I'd say Halo 2 at 60 FPS has the best aiming and shooting feel. It's definitely the most responsive of all the games, and seems to have the best tuning of aim acceleration and such. The reticle moves exactly as you'd expect it to, and it does so quickly. Player movement is also the tightest and most responsive, especially compared the Halo 3 which constantly feels like you're ice skating in a straight jacket.
Unfortunately the auto-aim and bullet magnetism are a tad on the excessive side, though. And by a tad, I mean a lot.
Halo 5 did feel better than the other titles though. Scoped weapons felt weird though, the sniper and the DMR especially.