I main AP top and am what I believe to be reasonably high elo, I'm in and around 50 wins up over my losses and que nonstop into DD competitors.
My playstyle is almost 100% AP Assassin centred.
I main Fizz. He's just too strong as an AP Assassin. The entire un-targetable mechanic as a game feature is just absurd in concept and in use. You can harass by using W>Q>Auto>E away, and you're DoT has just bled them out for around 1/4 to 1/2 their HP with very little danger for you. Add in the fact that you can further kite by sticking a smart-cast ultimate while waiting for your E CD you can almost exclusively crush all champions who lack low CD gap closes. Build: Boots Ring > Sheen > Ionians > Lich > Guise Radaon's or straight Rabadons > GShroud > FH = win game.
I've recently been delving into Evelyn play. She lacks a little power early and should be used primarily as an isolated waylayer and or bushwhacker until DFG/Lvl6. Lead out with DFG > Ult and you're already going in on a half HP opponent, enjoy the free pick-up. Absolutely 100% abuse speed shrines when you're in the jungle, a max stack W with a speed shrine buff sends you off at light speed, you'll outpace anyone other than POOTERSS. My only gripe is that you really need to watch out for Teemo, Brand, and Swain with Sweeper, their dots with the sweeper vision are a death sentence.Build: Boots Amp Ruby (don't go ring, your mana regen in stealth means that the mana regen from ring would be straight out gold inefficiency and the scaling rate of eve w/ pen makes the rush guise OP) > Guise > Kage's > DFG > here your build will get significantly more preference based, some rush Rabadon's for instagib, others go rylai for insured kill potential, either way, follow with a void if you get the time.
Now, although technically not being AP Assassins you are very probably going to want to delve into some Zix, Rengar,and Talon play. Rengar I see as a bit more of a bruiser so I don't necessarily play him as heavily, but Zix and Talon are powerhouse assassins at their finest.
Zix, I don't play him very often, it's usually in response to 3 or 4 straight jaycejaxyorickkassadin matches where I need a free win for the sake of my sanity. because that's what a wp zix is; a free win. ALWAYS evolve wings first, once you have wings you can jump any wall on the dom map (except maybe fountain wall?). And, for the love of god, do NOT build trinity. Zix is like jayce, you could build him trinity, and it's alright, but straight AD Zix ruins lives. At around 6, your Q will be hitting for in excess of 600 damage... WTFBBQHAX?! Evolve Q second, and depending on personal preference spikes or redzix last. Personally, I run redzix. Some people trash on this but let's analyze this for a second, you suddenly get a 3rd disengage and you now take 60% damage for effectively 3 seconds.... or you can have an AoE slow with your W.. the choice is yours however. Build: Boots Blade > Brutalizer > Phage > vamp > LW > Sanguine > FM.
Talon, think Zix for those who have dignity. The most self evident differences being he has no long range wall-bypass. However, his silence largely makes up for this at least in 1v1 scenarios. Don't ever forget that you CAN use your silence to a minion to get out of bad situations, I don't understand people who silence their pursuer assuming that will keep them from being caught with CCs/skills, when they can just jump to a minion wave and be gone. Your ult is a jukemasters dream. Press R, and assuming you don't eat an insta sweeper you just threw the entire enemy team into a panicky "where the hell is he" situation. one of my favourite combos is Rake>Q>auto>R> E to minions to escape, you've essentially just executed a mass nuke to premise a clean-up for your team. wp. Build: same as zix omit the phage until where FM stands in the build order.