Depends on your criteria. On the short term, AP Yi has the highest defensive steroid in the game on top of the fastest sustained healing effect in the game, which means that, during that few seconds, he can take more punishment than anything short of pure immunity moves such as Kayle, Trynd, or Poppy Ults.
Alistar also has obscene short-term damage mitigation with his max rank ult, but 70% straight mitigation is still less than 300 Armor/MR (which are 75% mitigation, before penetration), and his heal is far inferior.
For sustained damage mitigation, I believe Rammus wins since his rank5 Defensive Ball Curl gives 120 Armor/MR for 6 seconds with 14s CD. If you max your CDR (which can be done with tank items), then it's down to an 8.4 second CD. That's 71% up-time, and that doesn't include any other Armor/MR you've already built.
Health is cheaper to build than resistances, these days, so I'd give the advantage to champions with Armor/MR steroids in their kits over straight HP or HP regen buffs. It's all about Effective HP. I'd have to do the math to know for sure though.