Fiddlesticks is a beast. That being said, you need to know how to tame the beast, get it under your control and use it to vanquish your enemies with force.
Seriously, Fiddlesticks has one of the greates ultimates in the game, it's late game damage is just ridiculous, and it can just turn the tide of the teambattles. It also has a built-in mini-flash that WILL help you get to position.
This build is what i tend to get on all my fiddlesticks games, sure that it is not exact on all games (sometimes, when i'm better i just go straight for Rabadon's Deathcap, other times i will do Zhonya's Hourglass.)But still, most of the times this will cover it.

Pros

-He can dominate almost every lane he plays, if not CC'ed.
-His damage is unbearable at the start of the game, most people who fight your drain WILL lose.
-Perfect lane sustainance (drain)
-Can jungle

Cons

-Kind of easily countered (Ignite, Silence,Stun,etc.)
-Can't push for ****
-His main damaging skill is channeling.

The build is solid, you will survive with it most of the times, but there will be ocasions when it will need changes.
If you're facing a caster-heavy team, you may consider doing some MR, if a melee-heavy team, you could rush your Zhonya's Hourglass faster (that's also a great choice if the enemy team has a fed Karthus)
Get out of the base with a Doran's Ring, i will provide you with everything you need in the start of the game. Get Sorcerer's Shoes for some more magic penetration, and then rush Abyssal Mask (only if no one in your team made it/is going to), Then go straight for Rabadon's Deathcap (the bread and butter of every AP carry.) Morello's for some CD Reduction on your Dark Wind and Ultimate, and finish up with a Zhonya's Hourglass for extra armor and huge AP.
Later you can sell your Doran's Ring and make either a Void Staff for some extra Magic Pen. or another Rabadon's Deathcap for extra AP, it's up to you.

Why not?

Rod of Ages, it is too expensive and doesn't provide that much bonuses to you. You are NOT that mana hungry so you need catalyst, if you get it do a Banshee's Veil.
Also, don't get Archangel Staff, on most of the times you will not get even close to stacking the Tear of the Goddess. (you're not a spam-tard caster)

Maybe

You can consider some spell vamp on your build with Hextech Revolver and ****, i've never tried it but it is up to you.
Also, some sayed Rylai's Crystal Scepter is great, and i agree (because you are squishy, and a 30% slow on drain is great)

Bountiful Harvest is of utmost importance, it is your main damage and regen, it will keep you alive on the worst situations and will **** your enemies. So focus on leveling it.
Terrify is the great disable, it is what will save you from 1x1, help you escape, give you the upper hand on combat and help you nuke your enemies. it should be leveled second.

Why not Crow?
Reap is a fine skill. But that's all. It lacks the damage of your Bountiful Harvest, and it does basically the same thing that your Terrify does, without the awesome disable that make your enemies run around while you drain them up.

On the past, i was using CDR instead of AP runes on Glyphs, but as i've noticed, he doesn't really need them (based on the fact that his skill is channeling) so i've just adapted to some more AP (since i've got pretty decent Magic Pen. with my passive and marks.)
Mana Regen is just straight-forward caster, fiddlesticks is kinda of mana hungry, so its up to you.
i've also traded sometimes mana regen for armor/magic resist and it worked well, since i don't level Dark Wind on my build, i don't get a lot of mana use. It is rare the time that i don't get a kill for lack of mana.

Not much to talk here, it's just straight-forward offensive caster masteries, giving more detail to Blink of an Eye (CD Reduction on Flash IS definately a good thing) and Presence of the Master (MOAR)

Starting with flash, this is a MUST HAVE skill on fiddle, the mobility can help you re-arrange a not calculated ultimate, it can also save your *** when you're being ganked by that jungler. There's not much discussion to this, just get it.
Ignite is great for guaranteeing some kills on the laning phase, believe me, without it enemies WILL escape with nothing but a little dot of health.

Other skills to consider

Ghost: Ghost is great, it gives you speed and helps you escape, but flash is just better.
Teleport: Another viable choice, but i just can't see myself without the damage from ignite, or the mobility of flash.
Smite: Only if you're jungling
Clairvoyance: Unless you're some kind of CV pro, don't get it.
Cleanse: If you like it, get it, i still prefer Ignite and Flash.
Exhaust: Great for pinning down that enemy, or disabling that carrier.


Skills NOT to get

Heal: That's pretty much self-explanatory, your drain will heal you when needed.
Revive: Who uses that ?
Rally: That's not really an option for a caster is it ?
Fortify: It's not you job, leave it for the tank or the support.
Clarity: As i've said, you are NOT mana-hungry.

Fiddlesticks has a great lane domination, you can start nuking your enemy as soon as lvl 2 with your Terrify+ Bountiful Harvest combo, drain keeps you healed and fear guarantee that you can disable/kill your enemy.
Act like you own the lane, unless the enemy has Ignite/disables, Like Cho'Gath's Feral Scream that can cancel your channeling, you can dominate the lane absolutely.
Here's a personal tip. Crowstorm has a 1.5 casting time, as soon as you hit lvl6, use it.
I usually do it, and it WORKS, i'm not asking you to hide and do it. Start auto-attacking the creeps, and as soon as your enemy get close to the range of you ult JUST DO IT MAN, unless your enemy has a really fast reaction time, or flash he WON'T escape, and when you're by the side of that pesky Veigar, with the crows cawing on his ear, you just do your thing with Terrify+ Bountiful Harvest.

In the teambattles, don't be afraid to use your ultimate. I've seen fiddlesticks decline in quality just because they were afraid to ultimate, you must NEVER be afraid with fiddle, remember you ARE fear, you ARE terror, you ARE THE ****ING SCARECROW, ACT LIKE IT.
Use your ult to Dive towers, use your ult to kill single stragglers, use your ult to do your bidding anytime you want, believe me, in most times it will not be on CD when your team is gathering to battle.

There's nothing (and i mean nothing) more terryfing than the CAW CAW CAW CAW CAW CAW of your ultimate, use the fear of your enemies in your advantage, make them expect Crowstorm at any second,be unpredictable, That's the soul of Fiddlesticks, FEAR.

This guide contains:

A solid build for most games, with suggestions of what to do and what NOT to do.
How to act in lane, and in battles.
Runes and Masteries

Special Thanks to:
Chief Tiger's Fiddlesticks Build (it's been an inspiration for me.)
All my friends who tolerated me learning and then me talking about how awesome fiddlesticks is, special mention to: Malandropex, Castoor,joaobrbrasilsp, Barradas,RinMaroto,ZehVT,Fuguetero,Bambucha,Darkunstoppable,JofreBaguete,RogeerBR,Maulis and all of my friends who started me into this awesome game.

SS's and ****

September,17 - Guide published
September,18 - Guide Updated with some color and icons
September,21 - 1.000 Views, Thanks!