Honestly, Halo CE
It was my first Halo game ever, so there's nostalgia (my dad introduced me to it and played through the campaign with me when I was 12)
I love the aesthetic of the game and the "brutalist" designs. I don't know why, but I really like the low-poly models and the retro feel. I also love the soundtrack, the open level design, and how powerful all the weapons are (e.g the shotgun's range)
I'm the most familiar with CE by far. I started gaming on PC a few years before MCC was on Steam, so for a long time, all I had available on my PC were Halo CE and H2. CE ran better on my crappy laptop though, so I played that more. I've beaten the campaign countless times even on legendary, and it never gets old. CE is kind of my "comfort game"
Reach is a close second because Reach was the first game whose multiplayer I actually got into