Dude, I believe the MAIN reason why Halo 1 is better is because of just how richer it plays.
Bungie took the time to devolop a gameplay filled with nail-biting sounds and music that really make you fear some areas and make you feel like they're hostile to your presence or downright epic music highlighting every single one of those amazing moments and environment introductions, impressive or impactful set-pieces to intriguate the player into a battle filled with twists, tricky situations thrown right in your face that force you to use tactics, clever and shocking moments that impress everytime, and better AI. Every single encounter in the game is meticulously placed or sent to give you the most overwhelming, surprising, thrilling and exciting experience.
In the 2 sequels, in comparison, you just get into an area, enemies come, you shoot them. No twists, no set-pieces, no surprises: NOT ENGAGING!!! JUST BORING!!!