Hardline is a waste of time unless you are doing Hardcore or lone wolves imo (when kill streaks are generally shorter).
For TDM, Domination or Ground War, THE best setup all round that anyone can rape with is:
Scar
Silencer
Scavenger Pro
Stopping Power Pro
Ninja Pro
Any secondary grenade you want
Any secondary weapon you want, I tend to use the AA12 though as with scavenger it eliminates the low ammo problem, and once you get the grip attachment it's even better
Killstreaks, any 3 of the below:
Predator
Harriers
Pavelow
Chopper Gunner
Tactically I tend to go around the edges, move as far away as I can from my team mates yet watch the way they are facing on the map and then try to flank the opposition. Try this and I think even a mediocre player will be able to do well.