Fiddlesticks is easily one of the strongest, meanest HP regen heroes in the game. The health gain due to drain is very good (100% of dmg dealt), and in this build, i capitalize on that. Obviously, relying on 1 ability in order for a hero to work is a silly idea, and can be countered easily. This build however, using all of Fiddle's abilities in conjunction turns him into the biggest, baddest, AoE Nuke; to top it off, he is practically invincible while comboing.

side note:

I have written this build describing how things go in an average game, no feeding or anything, although that usually happens, just so you can see the true potency of this build while it isn't at full force. The principles of what to do when you are fed are the same as when your not, just remember that the most important thing is to play it smart. So try it, and have fun with it.

With this build about lvl 18 with average gold (not being fed), you can expect:
AP: 450ap at the least
37% passive Cooldown red.
Life Gain while Draining: 500+/second
Life Gain while Ulting and Draining: 80 life, per person, per second (including creeps) + an additional 500+ from the person you are draining.
On top of this, you are dealing 500 dps in a fairly large AoE and an additional 400 dps to the person you are draining. on top of that, throwing your crows right before the drain, procs another roughly 250 dmg with each crow to 10 people, with an added silence, and total life gain of about 1000. For the duration of an ult on 3 people (average in a decent 5's game, or 2 for a 3's match, and assuming no creeps are nearby) you are getting roughly 4700 life gain. That's right, see how the other team likes dealing with that, while taking at least 2.5k dmg each over that time? Your life gain per second increases for each nearby enemy by 400 during the duration, and likely killing that enemy, so, during this combo you are literally invincible. Scary for their team is, a silence wasted on you stops only your drain, and therefore had little effect in actually bringing you down, or saving their butts.

Pros:

During your ult combo on a group, you are unkillable.

After a fight, if you find yourself at very low hp, drain a caster minion and you will put a little cash in your pocket, and can run back into the fight at full hp.

1 on 1, you will never lose unless they have ridiculous spell resist.

Nearly max passive cdr means that the cooldown for drain is about 4 seconds, and it lasts for 5 seconds. . . do the math. INFINITE DRAINING!

Cons

The biggest, is that fiddle doesn't really have an escape ability for ganks, so really watch out for those as that is when he is weakest (3v1 without his ult) although he can 1v2 very well once lvl 5.

Somebody will likely try to ignite or executioners calling to slow your regen. (50% of 4700 is still 2350 life gain) big whoop

Their entire team might build massive spell resist: solution, have a team that does primarily physical dmg. If the other team stacks spell resist, the rest of your team will slaughter them, and you can still terrify, silence, and do mediocre damage.

The other team will not like you, and will run if they see you, making it difficult to get kills.

The other team may rage quit and will, therefore, make the game less challenging.

Masteries are simple and straight forward, grabbing ghost and exhaust improvements, and some nice cdr. Other than that, bonus xp and mana regen is nice and that is about it, everything else is pretty much filler.

Ghost and Exhaust are my favorites, although Flash with his Crowstorm is also very nice. I personally prefer Ghost/ Exhaust because Ghost right before your ult lets you chase people down, and, by yourself, using Exhaust and Terrify can keep at least 2 people inside your ult for the duration every time. Exhaust is also useful incase you find yourself alone against someone like Trynd. When he ults, he has no hp. no hp means no lifegain. Exhaust remedies this problem by reducing his dmg by 70% for the duration of his ult and allowing you to still kill him without dying.

Skill sequence is very easy. Grabbing Bountiful Harvest whenever you can, and a few levels in Terrify. between these two, you can get early kills very easily, and during a gank, Terrify lets your allies catch up and help out. Reap is nice, but usually isn't necessary until later. It won't help much with kills early game either.

The big parts of this build are CDR, and the all-important spell vamp. Grabbing Will of the Ancients is 80AP and 20% spell vamp by itself,so grabbing it as your first or second completed item means that even early game, you will not need to worry about health except when being ganked. (who doesn't in that situation?) Sorcerer's Shoes help with speed and also giving magic pen. Morrello's Evil Tome gives AP, gives you the mana regen you need, and Cdr: it is a perfect item. Grabbing a Hextech Revolver (into Hextech Gunblade after you are full build) gives you 15% more spell-vamp, which is very nice on your ult and drain.

optional items

Naturally, grabbing Mejai's Soulstealer is always an option when you are getting massive kills. I prefer to switch out litch bane for it.
Abyssal Mask is also a good choice, as the spell resist reduction stacks with your innate.

The rest is up to you, but this is the order I grab my items, and it works every time.

The big part in all of this, is placement. Knowing when to use your Terrify/ Bountiful Harvest combo can give you lots of easy kills, and when you combo them well with your ult, the game could be a short one. When a team fight is about to begin, get to a quiet, secluded, out of the way area from which you can ult in and raise hell before the enemy team can stop you. If they catch you while you are channeling, and silence/stun/disable you, you are SOL. your cooldown is going, and your ult didn't go off, so make sure you place it right.

MIDDING

Fiddlesticks is a strong mid champion, using drain and nice mana regen, you can stay in lane indefinitely. You can even grab the blue buff at lvl 5 without a problem. Some heroes to watch out for are Katarina, Miss Fortune, and Gangplank. Using their Health-Gain reduction abilities, they can slow your lifegain enough to push you out of lane. You may need to allow somebody else to mid if you can't handle them.
Once you are lvl 5, you can terrify the other mid as they get close and then just run up and drain them. This will usually take about half of their hp if you do it right, and multiple times will either get a kill, or push them out of lane.It is important to try to box out the enemy champion whenever possible. Stay in between them and the fighting creeps so they don't get experience and you do.

Kills they won't expect, and can't avoid

Once you are level 7, your drain can about match the damage output of the first tower, so feel free to show some balls and Crowstorm in on tower while the enemy hero is there. Immediately Terrify and Bountiful Harvest, and they are hosed if they are at 1/2 hp or less. Place your ult so you will have a quick escape route after the kill, and watch for mias before you begin, otherwise it could end poorly.

Ganking

Once you are level 7 and have maybe gotten a kill or two mid, you will want to start ganking other lanes. heading there after you have gone to base to buy, or gotten a kill mid is the best, but if not, just wait until you are out of vision and make your way to another lane. Execution is simple, just get behind them in a bush or behind a wall, ghost, and ult in. Picking targets is just as easy. If your ally has a disable, split it up and terrify one guy while your ally gets the other. At this level, your ult alone can kill someone at about 1/2 hp if you keep them in it the whole time. Other than that, drain whoever has most hp, or is the closest to you to assure the kill. If they are both low hp, I prefer to just terrify one, exhaust the other, and keep the both in the ult for the duration. Then I just drain whoever wasn't destroyed.
Grabbing the blue buff helps alot when ganking, because it allows you to not only have a shorter cooldown on your ult, but gives you a cooldown on drain less than it's duration. Allowing you to drain one guy, and as soon as he dies, drain the other immediately. The mana regen also helps you out with staying in lane. Using all of this, I usually don't have to b until I have around 2k gp and am about lvl 9-11. However, I never really make an effort to last hit creeps, which you probably will, so you will likely have more gold.