I don't mean the tanks that you deal damage to, they just have a lot of health. I mean the tanks that won't even be bothered by you. The one champion that comes to mind is Nautilus, but are there any more like him?
Alistar has the most base HP (2400) and during ult - 75 % damage reduction.
in b4 the tank definers [slayer-pantheon-popcorn]
with 
+ 2 situational items.
Unkillable and kill ur Squishies in 3 seconds.
Warwick, he's tanky, has a damage reducing spell, and has the best sustain in the game.
Definitely Poppy. She has a passive 20% of her maximum health shield on a 10 second cooldown, she has 15% increased armor and magic resist, even further increased by 15% for a total of 30% increased armor and magic resist IF she falls under 40% maximum health and she probably has the biggest disengage tool in the entire game with her ultimate.
karthus
with 
as runner up
#1 MOAKI #2WARWICK (With steraks fully stacked into frozen mallet) #3 Aaotrox (With steraks fully stacked into frozen mallet) #4 TAHM LENCH #5 ZAC
someone with duskblade. sits in a bush and rapes you with 99% slow
Just had a Nasus: 99% slow from duskblade and then came his E which slows you over time (up to 50%) .. so yea .. died quite often thanks to his buffed Q (1.5sec cd=500+ each)
with full build, and his ulty
Nasus and Cho (if cho has a crap ton of stacks
The more frustrating thing is how much absurd damage treeman deals on top of it.
