I still really enjoy the Halo 4 format.
LT Grenade
RT Shoot
LS Sprint
RS Zoom
LB Boost
RB Melee
A Jump
B Crouch
X Reload
Y Switch Weapons
This is the most in-line with past Halos, and personally I find that putting zoom on a left trigger would be a handicap--I'd imagine it'd lead people to zoom in way more often than they need to, and destroy any capability they would have with a BR, pistol, or espeically a DMR. By relegating it to the right stick, you still use it when necessary, but go without it unless you think you need it. Although, I do admit, this gimps the AR because I never use Smart Scope with it and so I always lose gunfights when I have it out--but instead I just switch to my pistol at spawn immediately, which can handle the job while I search for a better weapon.