Repair Item is probably the most useful one in a co-op game if things get tricky early on, but considering how easy it eventually is to just TP back to the surface and have Griswold/Adria repair/recharge stuff I'd say that realistically there's no considerable advantage to any of them.
Disarm Trap is largely useless. It would probably be my favourite though if the traps themselves didn't just flat-out suck.
The noblest of all dogs is the hot dog: it feeds the hand that bites it. DOOMBEARS II - Return to the Labyrinth: http://www.youtube.com/watch?v=yWdejK6yKiQ