No. He's definately not the most powerful. However, he's the most situational in my books, and the correct situations can name him the most powerful.

Take for if you had a Galio on your team. He ult locks all the enemy team into place allowing you to get free hits on at least 3-4 of them. This will make him extremely strong. Now imagine you have no AoE team locking ult. And you're playing a lot of off tanks / assassin-y type champs with dashes. With Twitches almost 0 escapes and super squishy nature he's easily killed and looks very weak.

Twitch also suffers from the fact that his abilities stack with AP but deal physical damage. Not helpful really.

Personally if you're in that situation I pick MF anyway. Her ult can deal similar AoE physical damage, she has an attack speed steroid, and AoE slow, a move speed buff and a passive (W) which deals extra damage. Her Q when can also be counted as a sort of Nuke like Twitches E. What she that twitch doesn't is an anti heal, and what Twitch has is slight invisibility.

Kayle + Twitch still op.

Twitch outclasses MF in every way.

Her ult deals much less damage in teamfight, and she's instantly CCed when she starts. Twitch has better AS steroid AND better AOE slow. Twitch has better MS buff and his innate deals much more damage than fortuna's W. Her Q deals much less damage than fully stacked Expunge.
As for heal reducing vs invis- 7sec invis allows you to move yourself before teamfight into right position without being instabursted or CCed by Skarners or bursty AP mages etc.

Twitch has highest AA range of all ADCs, and after buff you will rarely miss your target. AoE CC isn't necessary for twitch.

Also, twitch can outdps your MF in every stage of game, while having more utility and teamfight presence.

As for deadly posion, it might seem weak on paper, but i played against twitch as Shen- i had like 300 armor, 4k hp. When he killed me, damage recap said that he dealt 60% of his damage as physical (1.8k) and 1.2k as true damage (410 ignite+deadly posion). i was like WHAT?! It's even better than corki's innate.

To my surprise, twitch can easily 1v1 even Lee Sin, who is considered as counter to stealth champions.

Frankly, Im not sure the comparison to MF is entirely accurate. MF is a straight up lane bully - you are almost guaranteed (nothing is guaranteed in current meta where jungler determine the outcome :P) to win against any lane with a decent support. Exception might be draven. Twitch is a lot more vague - he can be a decent laner with a good support (and yes kayle is good for him), but he isnt that strong in lane. MF naturally falls off late game - its a price she pays for lane dominance - so comparing her late game vs twitch might not be fair.

Now to other stuff - 800 true dmg from twitch passive? You know its at most 48/sec (6 stacks). So you were fighting him for 16 seconds? Thats ridiculously long time. Yes its comparable to corkis innate - but its not that innate that makes corki strong (although it indeed is good) - also on shorter term corki innate is stronger . Btw MF with phantom dancer and maxed W would get around 30 attacks in counting double ups meaning 1620 magic dmg. Maybe its less on shen (you most likely had definitely more then 100 magic res), but on many targets (priority ones at that) MF passive can easily do more. Its 54 magic per attack (which is around 1.5 with PD - and using W/double ups takes it up to 2 or so) vs 48 true per second.

Twitch is ok i think - but he IS situational and at least so far i wont call him OP. Havent seen/played any game that would make me think otherwise.

I played him alot recently, and i can tell that best supports are:
1. Janna
2. Nunu
3. Blitzcrank

Janna is awesome- AD steroid for extra boom with your expunge, AoE CC, slow, additional AS... Awesome. Also this lane is very hard to gank- twitch will always stealth and janna will save herself with QER
Nunu is better than janna in teamfights- AS and MS steroid for twitch are outstanding. Also E destroys enemy carry's DPS.
Blitz for obvious reasons.

I had good score in every game, with like 70% win ratio.
My three recent games:
28/6/7 - i was flamed for being noob by enemy blitzcrank (he died in 3 shots every time)
17/4/2 - tried different build, but BC is still superior to IE rush
14/1/5 - quite hard lane in early stages (first 5 minutes - MF+Sona) but we stomped them after first recall.

Well he is never OP only things to remember how do use the stealth, on which skill order you use and how to keep skill set on order so you don't waste them.

Me and Zaul1z are pretty much exprienced persons to play as Twitch he is good on countering many other carries.

On my opinion Caitlyn is over looked AD carry just like Ezreal they both are squishy and easy to counter but Twitch has stealth what makes you feel terrified because you don't know where he will appear.

hes good vs ez now all he has to do is dodge ezreals Q's and twitch wins. Also if you fall alittle behind you can always roam mid, his ganking potential is stunning.

Yep, he's great against most ADC's, Ezreal was his biggest counter pre-nerf.
Ganking potential is, as you said, stunning. He has highest scaling and damage of all ranged carries, so why not?

Deadly poison deals somewhere between 5 and 10% of your damage as true damage.

Btw. downvoters gonna downvote because someone said that twitch is strong or balanced LOL

Edit: Just played Twitch in blind pick: My support was Zilean (he didn't used his ulti on me in entire game) and enemy had cait+heimer. They outlaned us heavily, caitlyn had 10/0 at one point. Heimer was just too annoying, each rocket took like 1/4 of my hp. I ganked enemy Lux, killed heimer few times in 1v1 and suddently got fed. I won game by 2 shotting enemy champions with 21/10/5. Ulti too stronk in teamfights. After i got my beloved black cleaver, Caitlyn was unable to defeat me in 1v1 even with items worth 10 kills and 60 CK.

Edit2: another game, Twitch+LeeSin vs Taric+MF. We totally roflstomped their MF, she was like level 2 at 10. minute. Carried game with 21/9/12, every enemy champion except mao fell in less than 3 seconds.

Someone finally said that,

Twitch is more than viable,
one thing i dont understand, why everyone consider Twitch a troll pick
Myb because due ''old'' Twitch... ?

But,
Each time i get ~3+ kills/ 0 deaths, enemy TEAM starts to LITERALLY babysit bottom...