Cassidy is definitely my favourite NPC, he's tough, useful and has style.
Myron is also awesome, for being such a lousy, whiny jerk. In a fun way. Handy too, what with the creating stims and such. Shame he's such a lousy shot, but you can't have it all.
Sulik is very good. For getting some good starting cash at the Den. If I play the type to sell my companions into slavery, I usually use him for saving Smiley and then sell him as soon as I reach the Den.
Vic's repair skill is the only thing the makes me even consider having him in the party.
Lenny: Mostly Harmless.
Goris: Good companion, definitely one of my favourites as well. A much better melee sidekick than Sulik.
Robo-dog: Very useful as a secondary combatant, but not much personality to it is there?
Skynet: I was so disappointed when I finally got that robot. Granted, it's pretty handy with a rifle, but I had figured it would have something more to say. Seldom, if ever, bring it along.
Marcus: Personality-wise one of my favourites, but his tendency to burst-fire his travelling buddies doesn't earn him any extra points. Also, his inability to wear any useful armor and the fact that he CAN'T FRIGGIN RUN makes me endlessly annoyed at him.
The Modoc Kids: The most fun I had from those was, after having kept Miria alive for a long time I finally got fed up with her and divorced her at gunpoint at Golgotha. My "Married" trait changed to "Seperated" or maybe it was "Divorced"
This was on one of my first playthroughs, no patch no nothing so it may have been a bug