This is how I play non jungle Fiddlesticks. This build is primarily for 5v5 and will get you a win if you have the right amount of clue to use your ultimate at the right time. I typically purchase these items but the last two are always decided by what is going on in the match. This character has a deadly early game and if he gets around 5 kills with a mejais it is very difficult to lose. This is not the sort of character you can come back from when you are feeding.

I have chosen three AP quints for early game damage with dark wind as well as the AP glyphs.
The mana regen runes are essential as they allow you to stay in lane for long periods of time.
The magic penetration marks do wonderful things early game with the darkwind rebounds combined with your AP bonuses as well as your passive.

I have messed around a fair bit with my masteries for a long time and I have found this is the best combination for this build of items and summoner skills.

Dorans Ring: Excellent item with AP bonuses and mana regen which complements the rune build.
Mejais Soulstealer: Great to take advantage of the early game feed you should always aim to get with fiddlesticks. Without plenty of stacks, fiddles drain is quite weak agaisnt tankier characters. Basically I choose this as it counteracts the nerfs they have constantly been giving fiddles drain due to the enormous amount of AP you recieve from multiple stacks.
Sorcerors Boots: Just love that magic pen.
Will of Ancients: This is the most important item in the build other than mejais because it is what gives fiddles drain its prenerf strength. Which is high damage and huge life steal.
Rabadons: Once you get this item your AP should be at 600-700 due to mejai's stacks and your dark wind should be doing outrageous amounts of damage.
Last Two items
If getting focused by magic characters or AP hybrids such as Akali/Jax/Katarina purchase an abyssal scepter and a force of nature (this works believe it or not the regen is outrageous with drain).
If getting focused by AD characters simply buy a Thornmail and a Zhonyas.
A good mix is to choose abyssal with a Thornmail/Frozen Heart.
Void staff if needing more magic penetration.

Dark wind for early ganks, make a rush for the brush in lane and rebound off the two champs, combine with ignite and normal attack and you will easily get first blood.
Drain!!!!
Terror at level 4 for an easy escape.
Get your ultimate as this it what will get you the multi kills you have always wanted. This is what will get you the 20 minute surrenders you aim for by playing fiddles.
Buff terror
Buff Dark wind last.

Flash to reposition ultimate or to make quick escape. Be careful not to cast too early or you may interrupt your ult (slightly embarassing).
Ignite combined with drain is deadly and easily will get you first blood combined with dark wind.

Ultimate, Dark Wind, Terror, Drain, Drain, Terror, Dark Wind, Drain = Triple/Quadra/Penta