Prior to Halo Combat Evolved, my PC FPS loves were Doom, Quake, Unreal Tournament, and Half-Life (and all its derivatives CS, DOD, etc.). Likewise, also great love for GoldenEye and Perfect Dark.
Halo CE was the right game at the right time, IMO. I had little interest in the Xbox until one fateful evening spent at a Halo LAN party. Maybe it was the booze or the particular group of players, but it was like a magical, balanced mash-up of the couch coop Rare shooters and intensity of PC shooters. I bought an Xbox and Halo the very next day and enjoyed YEARS of fun exploring the mechanics.
Now I will say that it's weakest aspect is basic deathmatch. The best, most exciting games in memory were CTF and other objective oriented games. The power of the weapons was tangible compared to other games. The simplified inventory system combined with the distinctive weapons and play styles guaranteed that K/D alone wouldn't win games.
Halo 2 ruined it all trying to be like other shooters.