Hard for me to do an apples to apples comparison since I never played Halo 1 or 2 online. Nothing compares to Halo CE system link though. I don't know why Bungie was so adamant about never just giving us Halo CE multiplayer online. Even with the Halo CE 10th anniversary edition, they wouldn't do it. I guess they knew that doing that would just make the weaknesses of subsequent Halo MP experiences more glaring.
I remember liking Halo 3 online quite a bit.
Halo CE did have MP on PC, and that was pretty entertaining, albeit incredibly unbalanced. I mean, the pistol was good on Xbox, but with a mouse? Holy shit.
I've tried to get into PC shooters, but something about keyboard and mouse play is just inherently less fun for me than the feel of a controller in my hand. I have no problem with kb/m for RTS or RPG games, but I just don't like shooting with that setup.
For me it depends a lot on the game to be honest, for many console ports, using a mouse and keyboard just doesn't feel right. I always try mouse/kb first though, since it just comes more natural to me in general than the use of the gamepad.