Love or hate him, Teemo has always been a fav of mine and was actually my very first champ I played on LoL since season 1.

Many people will wonder why the heck are you jungling as Teemo. He has a hard time ganking early on as he has no hard cc and is squishy compared to the more tanky junglers. However, Teemo hss two BIG advantages to most other junglers:

1: He is extremely hard to counter jungle once hitting 6 and provides wonderful map awareness.
As most people know, Teemo is notorious for his shrooms, but the biggest thing is, you don't need to even deal damage with them for them to be useful as the map awareness they give you is amazing, especially with the season 4 changes to invisible ward detection being so limited. All you need to do is shroom the 4 points where an enemy must go through to your jungle and you know exactly when he's trying to counter jungle you, making it easy for you and your team to stop them. Further shrooms can be placed within enemy jungle or along river to help counter jungle or protect teammates from ganks. Once late game hits, your shrooms covering the map will likely do more to figure out where the enemy is than your ward spamming support. Also as another note: the shrooms will help to find the mischievous invisible attackers (aka Evelynn, Rengar, Twitch)

2: Teemo's kit compliments incredibly well with Feral Flare.
The biggest problem I've seen lately with junglers using the Feral Flare is that they pick champs that have little to no synergy with Feral Flare. If you have ever seen a Lee Sin or a Darius jungle with a Feral Flare, you quickly notice how they become ineffective compared to their Elder Lizard/laning counter parts. This is not the case for Teemo. Teemo is built one of 2 ways typically, as a hit and runner ap burst champ or as an atk speed on hit proc beast. AP is often the main for him as, well, he's an AP champ by default. But if you look more at his kit, it lends itself very well to on hit procs as well. His e adds 50+ magic dmg and a weaker magic bleed to each auto atk, his q blinds the enemy for 2.5 secs (counter enemy FF anyone? =p), and his w lets him position himself really well for team fights or avoiding certain enemy atks. Lastly, Teemo is a ranged champ. This is very big for the Feral Flare because this lets you get the Runaan's Hurricane. That means in team fights, with the right positioning, you can hit 3 champs at once for a bunch while getting that wonderful FF passive heal 3x. This makes you incredibly hard to kill without a ton of burst or hard CC.

Pros:
Incredible late game dps and survivability.
Amazing map awareness utility.
Very strong team fighter.
He's Teemo (Global taunt ftw =D)

Cons:
Slow start
Bad early ganks
He's Teemo (People will want you dead, maybe even your teammates -_-;)

This is relatively straight forward, you want 2 things, to strengthen your ability to survive those heavy end game engages, and to speed up your building of the feral flare.

3x Atk speed quints
For jungling and on hit proc, tons of extra early atk speed just makes sense, and with the recent debuff to lifesteal and buff to atk speed quints, it just makes sense to go with the atk speed quints for this.

9x Atk speed marks
Same reasoning as with the quints, but if you feel like something different, feel free to use hybrid pen or magic pen reds. It'll give your individual hits more dmg, but you will also be getting healed for less as you atk less frequently. Your call here.

9x Scaling AR seals
You are a late game carry, and you will stick to jungle for the most part until you get your FF, at which case the scaling will be better than the flat armor. You could replace this with scaling health if desired, especially when against AP heavy teams.

9x Scaling MR glyphs
Same reasoning as with the seals. Can be swapped out for something else, perhaps cd red when against AD heavy teams to blind more frequently.

This isn't really a set in stone part yet, as I'm still playing with this myself. The biggest parts here are get the atk speed boost, the move speed boost, the buff duration boost (blue and red and extremely helpful on teemo as it give you cc for red and more blinds/shrooms with blue) and the boosts to jungle for defense and offense. Aside from that, the rest are up to you, if you wnat more dmg, more util, or more defense. I personally like more defense as that allows to reduce the effectiveness of slows for facing champs like nasus or nunu and helps provide early armor for jungling.

Unfortunately Teemo has a bit of a weak early game for jungling, so we will need all 5 of those reds for starting out. Also, we are building the FF so machete is a given. For the start until level 6, pick up the ward totem for defensive jungling. I can't tell you how many times I've caught someone trying to ambush me at red only to get killed by my team. =D
This can be swapped out for sweeping lens once you hit level 6 as the shrooms and FF will be all the warding you need.

From here, our first focus is the FF of course so rush that lantern. Afterwards some basic boots for movement are helpful but can be delayed if need be. The big items here are BotRK and Runaan's. Which to rush is dependent on preference. If you simply want to farm your jungle and stay away from engages, then Runaans will let you farm FF faster. However if you want to counter jungle or get into gank happy mode, go with the BotRK for survivability. Either way, you'll want the other item next. These will be your core items and should not be changed/swapped out.

At this point you should have your core items and have needed to decide on boots. The 3 biggest ones in my opinion are swiftness, ninja tabis, and merc treads. Each fits a different scenario. Merc treads are for AP heavy teams or hard CC heavy teams with at least decent magic dmg and little to no slow effect. Swiftness is meant for slow heavy teams and mobile/runner champs. This would include champs like nasus, nunu, ezreal, lucian, shyvana, etc. you get the idea. Ninja tabi would be for ad heavy teams, especially ones that are auto atk heavy. Important to note that ninja tabis will not be all that effective against on hit proc FF junglers like ww, but will still work well against the crit ff junglers like yi. This is because all that on hit proc dmg from FF and even from BotRK is NOT effected by ninja tabis. As far as boots upgrades go, I've personally enjoyed alacrity as this further increases the effect of Teemo's w. Feror can also work as it help you chase better, but might make roaming through jungles a little slower. I advise against homeguard in these cases as Teemo is already a fast champ, shouldn't be too mana hungry and you will have sustain out the wazoo so the benefits are greatly lessened on him.

Now for the end game items. I've personally been consistently using spirit visage and frozen mallet for a few reasons. For spirit visage, you greatly increase the heal effect of BotRK and FF, you get heavy magic resist which is great cause blind does jack to most AP champs, and you get a nice amount of health so when that global taunt start to kick in, you can survive more of the inevitable hate. For frozen mallet, once again you get tons of much needed hp making it harder to burst you down, but you also get some nice CC taht work wonderfully with runaan's letting you slow half a team by yourself and making positioning for that 3x atk and heal much easier.

In certain cases you may want other items. Against heavy auto atk teams, sure you got blind, but you only can blind one person at once. So since you got all that sustain, why not turn that advantage into a bigger one by having you own dmg hurt the enemy team back? Get a Thornmail! =p In causes were the team spams horrible slows on you or has heavy CC but no real magic dmg, get a zephyr to suppliment your swiftness or ninja tabi. Against heavy ap, get more MR with wit's end of course. For those heavy sustain teams like mundo, attrox, galio, or warwick, get executinor's culling to cripple their sustain. And of course, your Teemo, the people who don't love you want you dead with your head on a pike. So let's delay that as much as possible with a guardian's angel.

Remember, adapting to the enemy team comp and seeing who are the fed ones and dealing with them appropriately will greatly increase your change of being deemed OP.

I start with the e for the extra on hit dmg to speed up that first buff. Afterwards MAKE SURE TO PICK UP YOUR Q, if you don't have q at level 2, you will have a very hard time jungling for those first few levels. This is one of the big reason why Teemo what makes a better FF jungler than twitch. Afterwards, focus you e as the added dmg helps speed up jungle farming and take your ult when you can (more shrooms = more map awareness). Make sure to take the move speed boost at level 4 as that greatly helps moving about between camps speeding up FF farm.

From here though, it is your choice what to max 2nd, and it should be done with the enemy comp in mind. If the enemy is auto atk heavy or has a FF jungler as well, you probably want to max q first as this will greatly increase your chance of success in engages. However, if you don't need to worry too much about auto atks or are safely farming in your jungle you can probably just max your w second.

You're a jungler so smite should be obvious. However the 2nd spell can vary.

Flash: Gets you out of plenty of sticky situtations and can also help get the surprise jump when ganking. This is sort of the standard 2nd spell.
Ignite: Great against heavy sustain champs, making easy to secure kills for those difficult early ganks.
Ghost: As a jungle yi would, use ghost to quickly engage enemies along with your w and help stick you to them til they die.
Exhaust: Similiar concept to ghost in that it lets you close in better and also reducing their dmg at the same time, however lasts much shorter.
Heal: Can help provide a little assistance for early jungling and can help negate those horrible ignites that cripple your sustain.
Cleanse: This one is only meant for facing against ww, malzhar, or skarner for the annoying ult CC. As you need to keep attacking to stay alive, this will help you do just that.

You are a farmer, not a ganker, so your route should run through all camps as often as possible. The typical starting path I take is this:

Blue-Wight->Wolves->Wraiths->Red->Golems->Wraiths->Wolves->Wight then back.

To help make sure you have the health for this, a strong early lease helps greatly, but also proper use of skills and targeting. If you have a weak lease and took a bit of dmg, use your passive for the atk speed boost at each camp to improve your survivability. Always make sure to blind the large monster at each camp and just atk it until its dead. Afterwords alternate atks between each of the small minion to add bleed dmg to the mix with your e and sometimes red to speed up jungling.

If you are against an enemy FF, if you have a good idea where the enemy team is, feel free to mix in an enemy camp or 2 stealing just the large monster to slow their farm down and to speed yours up. This becomes easier once you can shroom the map as it makes it much harder for them to counter jungle you back. In this case, make sure you get the Runaans as the last of your core items otherwise you'll just clear the whole enemy camp counter jungling much less effective.

Always make sure to grab camps as you move along as this will help to strengthen your dmg and survivability.

Until FF is changed to be more gank friendly, your team will probably hate you for most of the game. To effectively build FF, you have to farm jungle like crazy, this means you can only really come out to get the easy ganks when you have red and the enemy is WAY over extended. Teemo is jsut to squishy early game to do much else and if your team demands more than that, just let them know that its better for you to farm your FF to wreck late game then to never get it and simply feed their lane.

This isn't to say that you can't help your team at all in the early levels. You can start shrooming around your teammates lanes to help them watch for ganks and you can counter jungle when noting where the enemy ganks. This is great for you as all you need is the big creep for your FF and you've done you job messing up theirs a bit.

Once you've gotten your FF unleashed and some decent items, you should start wrecking enough to gank well so long as you don't get too in over your head.

Come end game, your map awareness from shrooms and your heavy damage and sustain with added cc (from red buff or frozen mallet) will make you a god send in team fights. Just make sure your team goes in with you or that you can at least pick off the enemy 1 by 1 as they run away or you may risk getting burst down in an unexpected 1v5.

Also remember, hard CC and heavy burst are your worst enemies, so judge your strength against them accordingly. In the right set up you could easily 1v3 under their turret successfully, but in a different situation, they will just pin you down under it with you not having a way to fight back.

This can work wonderfully in team ranked as your team will plan accordingly to your limits as a farmer jungler. However, solo queue can be a terrible headache. Often times people expect the jungler to always be ganking their lane for them, even if they push to enemy turret and never ward just feeding the enemy team.

If your team plays overly aggressive and just keeps giving kills so that you can't farm your FF and become an effective part of the team, all that's gonna happen is FF and report Teemo spams. However, if your team plays smart, not even defensively, but just smart enough to know when to engage and when to back off so as to not constantly feed and lose lane, you should get the time needed to farm up your FF and become that force of nature that carries your team to victory.

There is also the problem that jungle Teemo is still looked heavily down upon cause no one builds this way so he ends up being you typical late game squishy with moderate dmg that does little more than surprise shrooms kills and blinds for teams fights. Because of this, I have had teammates purposely troll in ranked games because they thought I was trolling. Part of a reason as to why I started this guide so I could spread the good word of jungle Teemo done right.

So with that said, make sure to try this build out in bot stomps and normals first before attempting it in ranked, if you don't already feel comfortable with this play style on Teemo, you're gonna have a bad time.

I hope that this build is to your liking and that you may even expand on it on your own. One shouldn't remain close minded and conform to the current meta of who works and where they should go. It's the thinking outside the box that gives rise to the new metas the players eventually adopt and then become norm or at least another good reliable strategy. Should this build intrigue you give it a shot and maybe you'll appreciate the odd builds and styles more, but at the least hopefully this has given you the reader a little more food for thought before accusing someone of trolling for doing something different without considering the reasoning behind it.