Speaking from mostly a 1.13 perspective, because it's been so long since I've played the vanilla game.
1. Ice. He's just cool, and a highly effective merc at a reasonable price. Give him a quality machine gun and he can easily take out 2 enemies in a single turn.
2. Ira. Yeah, her voice is annoying. But she means I don't have to hire a medic at the start of a game. Binoculars and scouting also make her useful even when she can't shoot worth a damn. Besides, there's something rather entertaining about taking a random schmuck like her and turning her into a hulking beast that can clear out entire sectors all by herself.
3. Barry. Hire him at the beginning of the game and by the end he'll be a top tier merc at a quarter of the price. Compare him to somebody like Trevor, who starts with better skills and a higher salary, but within only a few levels has such an absurdly bloated salary you're no longing getting the proper value out of it.
And it admittedly it makes me a bit of a curmudgeon, but here's my least favorite mercs just to add on.
1. Reaper and Shadow. Huge edgelords. Reaper does look a bit like Mickey Rourke's character from Rogue Warrior, though. So imagining him saying some of that games more memorable quotes is pretty damn funny to me.
2. Every single wildfire Merc. Not interested in some dorks Eastern European OC voiced by a teenager trying to sound cool.
3. Manuel. Not sure where this guy came from, but I'm pretty sure he wasn't in the original game. Adds nothing to the original rebel crew and just talks shit when the other (Far more useful and interesting) rebels die. It is pretty funny that none of the other rebels seem to acknowledge his existence, though.