I usually plays with a 4 people party, after some testing that's the guys I prefer:
Cassidy: by far the best, can use the Gauss rifle and power armor and seems to score deadly criticals more often than any other NPC.
Sulik: he's really great when I pick him in Kalamath, but not as much in the end game, because I like to use big guns myself, like the bozar and the vindicator minigun, so I can't have him on melee because he will enter in my line of fire and if you give him a SMG he will spread bullets at me and the party too much often to be fun. So I just give him the .233 pistol. Overall a good NPC.
Vic: a not so good version of Cassidy, but still good. In the early game his repair talents are usefull for repairing the power generator at the toxic caves and for lowering the forcefields at the SAD. In any case he at least don't get in the way too often.
Skynet: I like the robot. He can't use armor, but the enemies usually don't target him (what happens a lot with Marcus) so I set him to stay close and don't have to worry too much with him.
If I go for a 5 people party, then I would include Marcus too. But I think he's a mixed blessing, because with a turbo plasma rifle in the early game he's awesome. In the Great Wananmingo mine he can kill as much as my character. But in the end game he's a PITA to be kept alive. Enemies tend to concentrate fire at him and he doesn't have the "stay close to me" option. Against the enclavers he's a temerity. To avoid his tendency of charging mindelessly and being killed I have to set him to attack "who's attacking me" and that seems to hurt his offensive capacities.
The other NPC's I really don't like very much. Goris and the dogs don't seems to be able to really help against the enclavers and will get in my line of fire all the time. I don't have many experience with Lenny, but he doesn't seems to be good enough at combat and Myron is only usefull for making Superstims...
Overall my choice of NPC's tend to reflect my choice of characters. I like characters with good offensive capacities, usually with Big guns and fast shot, so I need NPCs that can attack at range without getting in the way. Also I don't like to have to keep watching them all the time...