i think the gravity hammer is the best for melee but for range i'd say Pistol/magnum is the best and its the first gun you get to have. I love busting capps in grunts !!!!!
I like the covenant weapons, mainly the beam rifle and the brute rifle on Halo 2. The beam rifle has good ammo and can still kill people just as well as a human sniper and I like the zoom screen better, and a brute rifle has rapid fire that takes down sheilds quickly and deals a ton of damage, only bad thing is it overheats and runs out of energy a bit quick.
What I usually try to get in firefight is a pistol as my main weapon, and hammer as my secondary. The pistol can take out smaller enemies (grunts, elites, etc) while the hammer is saved for larger and more powerful enemies (chieftains, hunters, etc).
So I would say the pistol is probably my favorite, since it works against so many enemies at medium range.