When I was leveling I used Delvar all the way to 100, and then moved on to Ishaal, then Tormmok, then Leorajh and finally Illona, who I found horrendous to level since she tried to tank everything and just ended up dying if I pulled more than 2 elites. Now, I only use Leorajh. I find his ability very handy, even when I'm in my garrison. I just log in, right-click him, get my crates and reassign my followers to new missions, pick my herbs and log out. Easymode!
Tormmok I've found was the most powerful in combat, with Delvar being great for AoE. Ishaal wasn't bad either after his loyalty quest...but I still just stick with Leorajh. I don't think I'll ever use Illona again.
How about you guys?