My wargame loadout:
Depending on the playlist, Frag or Plasma. Usually frag, but plasma grenades are useful for BTB, for all of those warthog and ghosts running around.
Hologram
Firepower
Dexterity
The two guns I use for firepower are the BR and the DMR. I use these two because BR is better for mid- to close-range, while the DMR is good for mid- to long-range.
Spartan Ops loadout:
Plasma grenades; good for sticking those elites, hunters and promethean knights.
Regeneration Field
Firepower
Ammo
Same two guns, mostly because it doesn't really matter what I have in the beginning, since I usually trade my guns out for whatever the enemies drop.
(sidenote: I wish you could use your custom-skinned guns for set loadout playlists like SWAT and so forth.)