This will be my first build, and I know when people see you using this build they will freak out almost immediately. Mostly this build is meant for carrying a pug game of 3v3's, but i have owned in 5v5 several times.

These runes are just the ones I use, and they are basically straight forward late game ability power. The quints I used for immediate early game harassing. Spell pen, obvious choice.

Most people want a well rounded character starting a match, but because of drain it really makes up for any lack of armor/MR early game. I focused entirely on AP as you can see the number of offensive masteries, and the more damage you do the more you can heal, thus an OP Fiddle.

Every good build says its all about who you're playing, and they're right. However, I always take Will of the Ancients, Spell pen boots, and the item that most people will complain you have, Spirit Visage. The next three items do differ depending on who I'm playing. If they go extra tanky with MR, get the void staff 4th.
Lots of AD output, go right for Zhonyas hourglass.
The abyssal scepter combines nicely with his passive, and should only be used if they have more AP output than AD.

Now I've seen several guides with Mejai's Soulstealer, and I recommend staying away from this item for several reasons. First, knowing you have a few stacks changes how you play and makes you less likely to help in team fights, and run away more often. Second, stacking pure AP without any other passives does not help this guide due to the greatness of most other items.

Because we built Fiddlesticks with the idea of draining almost nonstop during teamfights, Clarity is the handiest thing around. I use it in ever match, and highly recommend it.

The second choice is mostly preference, but I usually go for Flash. Flash enables you to ult from a bush that is slightly too far away, and then flash right into the middle giving you extra range on your ult.

I've given Ghost a try, and it can be great for chasing after ulting, or just trying to get another crow out to get the last hit.

If you're playing draft pick and can see most of their players before you pick fiddles, Exhaust can be helpful with 3 or more AD characters.

You will get wrecked until lvl 3. It is a fact about Fiddle, because early lvl in this build you are not that powerful. Try to stay away and harass with silence. Drain does little to no damage without some items, and you should not try to tank through any 1v1 with it until you get Will of the Ancients.

After lvl 3, you can escape most any gank with your silence and fear, and can drain someone enough to be able to heal through their damage(1v1). When you hit lvl 6, try pushing your lane to their turret, then sneaking top/bot for a gank.

Ganking with fiddles can be one of the most satisfy things ever. As soon as they see you have your ult popped, they run no matter what they were doing. I always try to ult when they don't know I'm in a bush, or assume I'm draining a minion. Because they have the same movements for a crowstorm charge up and drain, try facing away from your enemy if they can see you.

If they cant see you, it is almost a certain kill. Ult on/near them then immediately throw a silence, fear then drain.

Or...

If you know they can tank most of what you're dishing out, ult in and just follow them using your silence to stop any stuns/etc and fear at the very last second then drain. Ive killed so many tanks/ MR heavy champs this way. Every time I get one of these kind of kills I get a rage induced comment.

Now for some laning tips....
Because most solo laners are ap, your silence will hurt them so much you can control the lane. Try and use it every time it is available, and drain minions when low on health. Make sure clarity is used whenever you have less than 40% mana so you can use it again when you need it without having to recall. Another good tip, while you are at their turret with plenty of minions, use your fear on your opponent to get a few hits on the turret without being harassed. Fear does no damage, so turrets wont target you.

Drain has a longer range then marked while draining. I've noticed Fiddle will continue to drain almost twice the distance as the marked area, so if someone is just on the border line of their turret, fear them and drain form a distance and it will harass them almost all the way to their turret.

As long as you have another player and Zhonyas Hourglass, you can ult in and turret dive, and still get a double kill. It seems ridiculous that his drain will save him from that much damage, but like I stated earlier no matter what someones doing they will run from crowstorm, making most of the time at their turret purely turret damage while you silence fear and drain the two turret huggers. Generally this technique is only for a slightly fed Fiddles, and will at the very least score you an assist.

Last tip, I promise.

Turn on them. I have turned on so many people with Fiddle, that I have honestly been deleted off of peoples friends list after a private match. Even if you have less than 100 health, after lvl 9 you can still turn on and defeat (1v1 only) almost any champ.

Make sure you can get to a bush because that's the only way this will be guaranteed. When chased into a bush with at least a little breathing room and silence and drain are off cool down, you throw the silence then immediately drain. Usually they will stay assuming you are much to weak to live, and just drain them while they can only only auto attack. when the silence is up make sure you you haven't been stun/etc to stop the drain. If you do get stopped while in drain make sure you fear immediately and move. If they are dumb enough to keep chasing just silence, then drain from the bush one more time and its a good kill.

I know many people have done Jungling with Fiddle, and this build is not meant for jungling. However, drain at lvl 9 will allow you to solo dragon and clear all the creeps. I only jungle when being harassed by someone with a further range then fiddle, or if I really need to carry a team.

Silence, Over Powered AOE Ultimate, fear. All of these can save any teammate, and decimate enemies.

The only huge problem with this build is that it really doesn't help much when a 5v5 match has gone on past an hour and a half. I don't do ranked too often and I have only gone into a match that long a few times, and this build needs to be tweaked for EXTRA long games.

Any kind of ability that will stop Fiddle from doing drain is very annoying. Make sure that drain is used after any kind of crowd controls have obviously been used. As you could imagine there are plenty of characters that can destroy fiddles, so be careful.