Halo 1 was the first game I ever played online. I played a lot of it.
But I was a very different person then, and replaying it now reminds me of how stupid I was (because I got into a clan that was very based on that kind of person) and the internal ick just his a fever pitch and ruined the game for me.
I never played 2 since that was on Vista and I never went back to it after finally getting Win 7.
After that the series just felt tainted to me. When I first tried it on a console it was really weird with the control setup. I was very used to hundreds of hours on Perfect Dark with the default controls. Having a second stick and rearranging what hand controlled what bent my mind in knots for a while. But it ended up making more sense that way (as you can easily tell by how much it caught on- not saying Halo pioneered it, but it was my first experience with it).