Constitution.
And actually, it wouldn't have anything to do with my personal class preference. Con is my number one priority as a:
Akashic, barbarian, bard, champion, charismatic hero, cleric, courtier, dedicated hero, fast hero, favored soul, fighter, greenbond, healer, hexblade, mageblade, magister, marshal, monk, ninja, noble, ranger, rogue, paladin, samurai, scout, shaman, shugenja, smart hero, sorcerer, sohei, spellthief, spirit shaman, strong hero, swashbuckler, tough hero, unfettered, warlock, warmage, warmain, witch, wizard and wu-jen, among others.
I like to have characters with at least some chance to survive. Hit points and fortitude saves? I'm there.