Well, there's many different playlists and many different maps. Each one requires a certain Loadout and playstyle to be as efficient as possible. I like to play Big Team Slayer so I usually go with this Loadout, since Ragnarok is the most often picked map.
Primary DMR
Secondary Light Rifle/Assault Rilfe - Depending on the map
Frags
Hologram
Firepower
Awareness/Faster Ordnance (Forget the name)
I choose DMR for the obvious reason, big maps need a better scope.
Light Rifle because on maps like Ragnarok you go through a lot of ammo if you don't die often and it's handy if you find yourself needing one more shot. Assault Rifle if you need something for close range.
Frags because the blast is more useful on larger maps since I don't expect to be getting sticks on anyone that far away.
Hologram for general usefulness, a lot of people seem to be skipping over it since it was so useless in Reach but for some reason in this game people almost always try and shoot it. Helpful for offensive and defensive playing.
Firepower because of reasons stated above.
Awareness for dodging those nasty ghosts when they think they've got you off guard and for the occasional sneaker. Faster Ordnance because those heavies can make a big difference in larger, longer games. Taking out the teams banshee/mantis/tank etc. makes them take a huge hit in score.