For me, ME3's indispensable companions would be:
Garrus. ME's signature character, IMO. I've been a fan of his from the beginning.
Ashley. My ManShep's LI (and he did not
cheat on her in ME2
), plus I just think she's awesome.
Mordin. By far the best of the new characters introduced in ME2.
Tali. Don't hate the character, hate the fans.
I'd include Wrex on that list, but I prefer him on Tuchanka, uniting the krogans and reforming their society.
As for the suicide mission, in my only completed playthrough (as my ManShep), I got everyone through, but I'm tempted to go back and agree to let Jacob run the solo op, just so he'd get killed. Not that I dislike Jacob or anything, but it did feel wrong to not have someone
die on the mission.