Halo: CE: the Pistol.
Halo 2: Dual-weilding SMG's.
Halo 3: The Spartan Laser.
Halo 3: ODST: The siclenced SMG.
Halo Reach: The DMR.
Halo 4: The Magnum.
Halo 5: The DMR.
All-time: The Halo 5 DMR. I think Halo 5 in general has the best weapon sandbox of any of the games, and with the current DMR I am a formidable force at any range. I prefer single-shot weapons over the three-shot burst any day of the week. It also feels and sounds much more powerful than the BR.
Edit: My favorite req would be the Nornfang. I love using the sniper rifle, and the Nornfang is a one-hit kill anywhere on the body. It is also good to use against vehicles.