I had this conversation with a relative and now I am curious as to what other think. He said his was Lydia of Skyrim fame. I said mine was Imoen from Baldur's Gate. Imoen is fun, loyal, and actually goes through significant character development.

So who is your favorite NPC champion from a video game? (someone who can join your party).

Many of those. But Planescaoe: Torment's Fall-From-Grace is the first one to come to mind. ("Simple minds, simple pleasures." "Time is not your enemy - forever is". Yes, the Sensate succubus). I really enjoyed listening to her conversations with TNO and the others.
Oh, and... does Vergil from DMC3 count? He does join your "party" to battle Arkham.

Gavin Mor, an NPC mod for Baldur's Gate. Mainly for his excellently written romance, but he's all-around believable, likable character with little baggage. I just don't like to baby sit drama queens, and RPG characters tend to have plenty of "issues".

Minsc and his miniature giant space hamster, Boo. I can't possibly fathom a playthrough of a Baldur's Gate game without them.

I liked almost all of the companions in DA: Origins which was the last Bioware game that felt like a RPG. Allistar was pretty funny.

Crow from the longest journey
Annah from Torment
HK-47 from KOTR.
Arcade Gannon from FO:NV

All characters with funny lines or twisted personal pasts...

a couple of amusing ones from BG II:

Jan the Gnome - for his constantly taking the piss out of the paladin knight Keldorn

Edwin - for his side quest

My pet in Torchlight. Always loyal, holds my stuff, runs back to town, and turns into all sorts of crazy stuff.

Count me in for Minsc as well. I think somewhere in the middle of Baldur's Gate was where I changed from a guy who liked to play computer games from time to time, to a gamer. Why? Well, you know. Camaraderie, adventure, and steel on steel. The stuff of legends! Right, Boo?

Ashley.

Kidding, Ashley's absolutely horrible. (She's from Resident Evil 4 if anyone's wondering.)
It's been a long time since I played the game, but I vaguely remember Alyx Vance to be a very decent NPC. Not only is she a woman who is portrayed as a person (and not like a screaming, hysterical idiot like many women are,) she also wears normal clothes (whoo!) and is actually useful, if only because she doesn't run into your line of fire and commits suicide or murders you by firing at someone directly behind you and completely disregards the fact that you're there.

Double H

"60mm cannon... Zig-zag progression. Carlson and Peeters, page 39."

BG - Xan :D It broke my heart that I couldn't recruit him in BG:EE because I wanted to bring all 3 new characters with me, and there were no more space for him :( I liked Minsc too, but I never recruited him, because I didn't like his witch (and my PC was always a mage anyways) :/

BG2 - Anomen, because he was my first video-game romance, and this just made me like his character a lot. (and he treated me a lot nicer than the guys in school...)

From Dragon Age I liked everyone, except Leliana, because she was a bit too... pious? Childish? I can't really explain, I just disliked her, liked everyone else tho.

Skyrim -Serana, because unlike the other companions, she actually has a personality.

My favorite companion is Nordom from Planescape Torment, because talking with him was very innovative and non-human characters always are a welcome change. Moreover it's cute how he rationally analyzes everything.

...who I would have liked to have as a companion is the lizard man in Arcanum since I love the lizardfolk but unfortunately you have to choose right dialogue options and do everything in the right order to have him following you.

Morte, of course :D