This guide will be showing you how I play Fiddlesticks on almost every game (5v5). This build will be most effective against squishy characters just because of the amount of damage dealt. You'll might be able to carry game on your sholders.

Reason why I picked all of AP runes is:
1. Fiddle is really good as AP carry
2. It does stucks really nicely with Rabadon's Deathcap

Remember, to play effectivly with this runes you must have blue buff

Summoner's Wrath: stucks really nicely with Ulti, chasing and running from enemies
Mental Force: stuck with flast AP quint runes, gives to cutting edge in early phase
Sorcery / Intelligence: 10% cd reduction
Acrance Knowledge: 10 + 10 fiddle's passive of magic pen
Summoners Insight: 15 sec reduction of flash, can be very useful
Expanded Mind / Meditation: gives mana witch is vital
Greed: Fiddle is really bad farmer so greed and kage's lucky pick sucks well
Runic Affinity: Blue buff
Awareness / Sage: Most imporat is that you get 6lv before enemys
Mastermind: Flash and Ghost get big reduction

Start up with Doran's Ring, next rush to Kage's Lucky Pick and Boots of Speed.
Than depends how well are you doing. So if you are doing really well in that case buy Needlessly Large Rod, or if you are sucking in lane buy 2 Doran's Rings (they give great sustain)
After that rush to Rabadon's Deathcap and Zhonya's Hourglass cause they will be you core build. When you are done with core build than depends on who is most fed on enemy team.
If it is AP that is fed buy Banshee's Veil, don't buy Abyssal Scepter cause you have more than enough AP and that MR isn't that mutch (you could buy Abyssal Scepter only in late game when you need to lower their MR)
and if AD carry if fed than it not cause any problems to you.
In matters of late game or you are fed like hell after Bangee's Veil I strongly advise buying Rylai's Crystal Scepter (stuck really well with Ulti and gives you alot of HP)

Flash: It can be used to chase and blink away from enemys, when you have Flash and Ulti ready you can dive under tower and get away safely with Flash

Ghost: Movement Speed Effect stacks great with Ulti used as escape of offence tool,
no matters if you fail Ulti, Ghost help you to change that fail into win as you chase enemy champion down

Terrify: is often used like a stun, but can be use as escape tool
Drain: is you main spell, use it wisely (dont start fight with drain and watch out for CC)
Dark Wind: is good poke but real use of that skill is when Ulti pops out.
Crowstorm: is awesome Ulti cause you can dive under tower with it and most importantly never initiate team fight with it

Remember; Your Ulti has long long CD in you don't have Blue Buff so I would strongly advise useing it team fight and ganking never in 1v1
When casting Ulti: shill sequence should be R - E - Q - W (now to explain: When you seccesfully menage to cast Ulti throw E cause you will silence them for 1.2 sec and most people will be in panic so you will be able to see were are they going, next skill is Q just another CC that gives you time to Drain enemy with W)

A key to victory in team work, as fiddle you must patiently wait and position you self untill team fight starts. When someone initiate fight you must go and kill squishy enemys like AD or Ap carrys. It is most likely that they will focus you, in that case you would have Zhonya's Hourglass. Do not go in team fights in you dont have CD

Pros: great solist if enemy doesn't have CC, blue buff makes Fiddle unstoppable
Cons: bit squishy, bad farmer

There is much more to Fiddle, at least im playing this way and I think that this is the best build for him. Please comment and thank for reading.
Keep being updated, this is my first build and as time goes I will update this build when have time, again thanks for reading.