Not Updated For Current Season
I have been playing LoL for close to a month now and thought that by now I should contribute back to this site. This build is dependent on DPS considering your team has a good initiator or tank. You will be outputting massive magic and physical damage per hit as well as making your target weaker every hit. You will be taking out tanks really quick. This build works great for either middle or side lane.
Generally, you would want attack speed for max DPS and health for survivability in battles. With this build, you won't reach max attack speed unless your teammate has a Stark's Ferver and that you came out of your champion passive, Camouflage.
To maximize damage output, you would want to be able to chase as well as slow them down at the same time; thus installing points into utility and offense will be the most important.
**Offense Tree: You might be wondering why I chose Critical Chance over Critical Damage in the Offense Tree and here is why: this build is NOT for critical damage, rather it is for regular hits that will lead you onto an onslaught. Then you might be wondering "Why not cool-down reduction and archaic knowledge for the 15% magic penetration then?" You won't need either because you won't be spamming your skills. Also, your Toxic Shot is not a spell, rather it is a passive right when you get it.
**Utility Tree: Why allocate points into time spent dead rather than regeneration? It is because you won't need mana or health regen. You won't be spamming skills or tanking for the team, so it is unnecessary. By allocating points into higher experience gained, you will be hitting level 18 at a quicker rate than most as well as having a more wholesome effect of your Greater Seals of Vitality.
Maxing out Toxic Shot is a must as it gives not only more DPS, but pushing power as well. You will be able to push your lane out much easier by intoxicating the minions or enemy champion regularly until the player runs back to base to heal up.
Move Quick as second skill to max is a great idea because it will allow you chase down enemies at a much quicker pace before they run to a nearby turret. This skills gives you the ability to harass the enemy champion(s) with more ease.
Blinding Dart is maxed out last. Why? Because as Teemo, you usually won't be confronted 1v1 majority of the time. There is usually another player backing up the person that is trying to kill you. Though, if you are up against a Master Yi who relies in auto attack, maxing this out 2nd could be situational if he gets fed.
Ganking usually don't occur much in early game as most action begin in mid-game, so there is no need to rush on getting your mushrooms for sight or slow. Be sure to spam mushrooms when the it is end-game. If you are losing, spam them inside your jungle. If you are winning, spam it in some key areas of both team's jungle.
Berserker Greaves: Great as 1st core item for attack speed to push lanes quicker and to do more DPS. Even if the other team have many disables, getting Mercury's Treads is NOT situational. Don't get Mercury's Treads. As Teemo, if you can't get away with your current movement speed, then you won't get away with Mercury's Treads.
Malady: Great as 2nd core item for early game as it is easily obtainable. It deals 20 magic damage and removes 6 magic resistance per physical hit. This is a great item for Teemo as Toxic Shot is magic damage and so is Madred's Bloodrazor--the next item you're aiming for.
Madred's Bloodrazor: Near the time you get your recurve bow and pickaxe, the game should already be in mid-game. Tanks would have about 2k-3k hp by now. This item is great for taking out tanks as well as the attack speed most and damage that you are looking out for. Remember that this build is mainly DPS that is focused in both magic and physical damage.
Wit's End: This item is a great addition to your DPS as Teemo. By now you should be outputting around 400-600 damage per second with close to 2 attacks per second and red buff. This is great for taking out tanks that are mana-based as well as the casters nearby.
Black Cleaver: Another great item to weaken your target. Not only does this grant you a massive boost to your physical damage, but it also makes better use of your attack speed. You can bust out 5 shots to maximize the effect of -60 armor in seconds.
Bloodthirster: The game should be close to ending by now. This is a great final item to your core build as it gives you the lifesteal that you long for throughout all those battles and damage that you need to complete this build. You should be dealing about 600-800 damage per second now and perhaps even 900-1000 with Baron Nashor and red buff. Upon completion of this item, game should be over in minutes; either you winning or losing. Usually games should end before you get this final item.
I have tested this build in numerous 5v5 and carried the team majority of the time. This is my first build. Thank you for looking it up.