Probaly the tank companions if you talk level 50. even while leveling tank companions aren´t used much.
True. On my Sage I only brought out a tank character for the big boss fights. Otherwise Nadia is quite happily trashing through heaps of mobs.
On my Scoundrel I've been using Risha ever since I've got her. Even class quests were easy to do with her along and a few levels below quest level. Helps if your character is a healer