...To play against, anyway. What is the best way to counter Fiddlesticks in mid lane? And which AP champions do well against him in the laning phase?

Hard CC works well. Especially single target abilities with travel time (Kass, Annie, etc). He will fear and then drain, so by the time he fears, your CC is already in route. His drain will be canceled pretty much the moment he casts it. Ignite will also completely shut down his regen.

If he is jungle make sure to place wards at/around his jump locations. That way you will have vision on him before he ults.

If you feel comfortable doing so, other ranged skill shot CC will be just as effective (Brand, Cho, etc)

Once late game comes just don't get caught by a fear, as the enemy will quickly pile on you.

CC does work incredibly well, or someone who can match his sustain. Vlad may do well against him with his pool for his ult, and his Q for regaining health. Brand would do well, or Chogath, as KingSilas said.

Fiddlesticks falls off a lot late game anyway.

If you are facing a Fid mid, make sure you are calling for ganks. He has no escapes besides summoners (unless they are an idiot and use an ult to escape?). Just don't dive him, his fear has an OP duration, his sustain should keep him alive, and the whole time you are feared under the tower you've just taken 3 shots from it.

i know akali does well vs him from my own experience as well as viktor but hey hes underused and doubt anyone wants to try him

^ he's free right now

And I imagine an akali will try to harass, become feared, get drained, and then die in her stealth.

If I was fid I would just harass with Dark wind to around 3/4 hp followed by his combo. When she runs I would simply ult onto her twilight shroud, finishing the kill. She has strong harass, but fid can easily a)sustain the damage by draining minions or b) just fear her any time she comes in.

His ult + Zhonya's = 2 free seconds of high damage to all in the area. Even if Fiddles dies, he did his part. And if he doesn't die, dat fear op.

Ignite IS a good counter though. And if you can't stun him after fear (like with a stun-in-process) then merc treads are always good, that way you don't get feared as long.

But yeah, Vlad is a bad idea. So is Akali. They can't stop his drain.

Pretty much any mage with an easy way to break a channel will beat fiddle, and he'll beat any mage who can't in most straight-up fights. Probably my best choices for going against him would be Kassadin, Diana, or Cho'gath, since they'd outscale him pretty hard. His weak pushing also makes him vulnerable to mages who can't really fight him but can just push the lane from long enough range that he can't stop them - he'd be hard pressed to stop Zyra, Morgana, or Orianna from just pushing and ignoring him, for example, even if they'd probably lose any trade that isn't an outright fight to the death with ults.

About the only mage I can think of offhand who Fiddle beats mid really handily is Vlad. Nearly anyone else can either outpush him or cc/kill him when he tries to drain.

I have play him but if you know him you can counter him with any ap mid really and all it take is play safe less you go ahir or ziggs who just can zone him out and out range him.