It's really not something you can generalize.
Bears are the squishiest, some claim, yet many top guilds use them for world firsts so they clearly aren't 'hard to heal' in some people's opinions. Everyone keeps mentioning pally self heals but I think you're overlooking just how much DKs self heal now, it's ridiculous. Warriors don't self heal and still aren't difficult for us to heal.
In the end, the answer is "Any tank that understands their class, itemizes properly, and uses CDs when necessary".