I'm almost through the campaign of Marvel Heroes with Cable. I'm really still digging it. It scratches the Diablo itch really well. I did break down and pony up some money to unlock more bank spaces for crafting materials and a few characters, but it's still less than I would pay for a full retail game. I'm finding it fun still. Though, here are a list of the villains I've beat, see if you can guess who was the hardest.
Magneto
Blob and Toad
Venom
Rhino
Mole Man
Green Goblin
Living Laser
Elektra
Kingpin
Bullseye
Shocker
Out of all those villains, the freakin' Mole Man was the toughest. He constantly spawns moloids and hits like a mack truck.