I've played about every Halo game released since combat evolved on xbox, and probably from the list provided is the original

I have sooo many memories when Halo 1 was out and people were talking about halo 2 was about around the time when me and all my friends would play online through xbox connect. That and lan parties, plus a great story line made one of the most memorable games I have played in my eyes.
Halo: PC and Halo 2 come in a close second because you could play halo 2 online with xbox live, and it never ever got old (and it didn't even have as much dlc as any game now adays) and Halo: PC because of the mods.
While the graphics have improved, I just have never had as much fun in any of the halo games as I did in the first one.