This Guide isnt meant to be allmighty, but still strong.
I play Fiddle like this all the time, since it provides me with massive health restoration through all of my skills.
I dont play this Fiddle on mid-lane. Usually i play alongside a caster on top lane so that he benefits fro Will of the Ancients too.

Pros:
Massive Health Regeneration
Enourmous Power due to Magic Penetration
Almost unkillable in 1v1 (excluding Tryndamere as an enemy) due to Drain + Spell Vamp

Contra:
Most likely first target in team fights
Not really fast at running
Massive Mana consumption > needs Blue Buff

I like to play this fiddle alongside a Malzahar player.
If you yourself as well as that Malzahar player are buying Will of the Ancients, the two of you will receive the Aura from it 2 times. This leads to +60 ability power, +50% spell vamp (instead of only half the vlaue).

I also like having a Shaco on the line because of the Fear provided by his Jack in the box.
In the end, Champions who benefit from spell vamp are a good choice for mates as well as Champions who are able to slow/stun/hold/fear, so that Fiddlesticks can take his time with Drain.

i use Drain for my first ability since its the strongest skill of Fiddlesticks and restores his health.
Even on level 1 it already deals 300 damage if you manage to stay in range for 5 whole seconds. That might be hard, but if you manage to stay in range for even 2 seconds, your enemy will feel the threat and will retreat.

Next i max out Terrify to maximize the effect of drain. The duration of Terrify and a well placed Drain is a combo that no one would want to take.

Dark Wind might be a good spell to damage your opponents but in my opinion its best effect is the silence. But with my strategy i have no need for an early silence.
later on its a good skill to keep your enemies from stopping you while channelin your Crowstorm. You could do that with Terrify too but you would want to use it while Crowstorm is acitve so that your enemies cant come out.

i took Clarity as my first summoner spell because in the beginning Fiddlesticks burns his mana rapidly. It might be not that much of a help in the late game, but if you manage to stay in the line for a long time, you can farm enough exp and gold to make up for it.

I take Flash for my second summoner spell to maximize the effect of crowstorm. It might be not the best choice to take a summoner spell for the sole reason of maximizing the effect of your ultimate and some might say that i should better use exhaust but thats just my way of playing.

So after all you should keep in mind that Fiddlesticks + another Mage is almost overkill.
There was a game where i had stats of 1/9/3 on my line after we got ganked heavily. But as soon as I had Will of the Ancients, I turned the table to 18/12/24.

That item maximizes you life drain incredibly.

Note that fiddle doesnt need the ability power like others, he benefits far more from magic penetration (at most from flat magic resistance reduction).

Try it out and have fun.
Hope it works for you like it works for me.

Well there are many options for runes wich would be good for Fiddlesticks, but i lean on Magic Penetration.
Fiddles strength comes out better with magic penetration than with ability power because his scale isnt really the best.

You might want to take some Mana Regeneration runes with you because Fiddle needs it through out the whole game.

My masteries are 9/0/21 taking the ability power and magic penetration of the offense tree and leaning heavily on mana reg and cooldown reduction.

For items i prefer building Will of the acients before anything else. Its a great item in combination with drain because you can almost every time restore your full health. I take some magic penetration items like Abyssal Scepter and boots. Sometimes i take the Lv5 boots because a Hyper Speed Fiddlesticks is fearsome for everyone.
My other items slots will be filled with abilit power. So of course i take Rabadons since almost every caster needs it. Rylais Crystal Scepter is a good choice too since you are able to slow your opponents during your Crowstorm to make it harder for them to get away.