Other good combinations:
Rocket Launcher and Sniper Rifle
Pros: Amazing damage, great at long and medium range
Cons: Very little Ammo, long reload times, buggered if you're caught up close.
Shotgun + Pistol
Pros: Another powerhouse, Pistol has good range with scope.
Cons: Long range is an issue, small clips on both.
Pistol and (anything)
Pros: Pistol is stupidly overpowered, so use it with anything.
Cons: Small clip, long range is crap
Warthog/Ghosts 1 man
Pros: Instant kill when run over, Ghost has Plasma weaponry aswell, fast, absorbs some damage.
Cons: Warthog Driver's ramming has no range at all (obviously), both prone to rockets, ghost easy to dodge.
And finally:
Sniper Rifle and Pistol
Pros: All ranges covered with great damage and accuracy.
Cons: Small clips, sniper rifle leaves you prone to a 1 hit kill melee from behind.
EDIT: Lucipher, I don't like the tank as much as the warthog it's slower, has a minimum range and though is can fire two guns at once, the turret is slow enough that in any other transport you can strafe it without being hit once.