Halo 3
To this day I adore the way Halo 3 looks. The clean colors. no bloom, blur, or shiny metallic surfaces. Just clean colors that popped and were easy on the eyes. Maps like Valhalla, Guardian, Avalanche, etc. are still better and more memorable than anything I've played in Halo 4 & 5 and a good amount of why I feel that way is because of the use of color and overall map design.
It isn't just the layout of not just the multiplayer maps either but the campaign levels themselves. Open enough to be a sandbox while still being able o lead you down a path. I played 3's campaign many times and each time my friends and I made it a different experience. Not to even mention the sound design, music, or the mechanics of the MP, which practically changed my life. Halo 3's MP was fun.
I miss the days of Halo 3. BTB CTF Rat's Nest, Avalanche, and such will always be with me.