Basically, if you want to get consistently good scores, the AK-74u, with Grip and/or Rapid Fire, is your best friend - ESPECIALLY on small, close-quarter-combat (CQC) maps. And as much as you may want to snipe, don't, because on Black Ops, it just doesn't work.
I would recommend KS which don't require much input, such as Spy Plane, Blackbird, Dogs, even things like Napalm Strike, or Mortar Team. Killstreaks such as the Gunship require great player input, and may be very fun, but they distract you from the objective, and of course, shooting people.
If you're playing on a large, open map, with no alleys, or easily accessable flanks, you may want to use a long-range Assualt rifle - I find the G11 most effective for long range, but you could use pretty much anything with a high accuracy, such as the Famas. You could probably use an LMG if you don't plan on moving much, but they have very low mobility, so choose wisely.
If you have trouble with accuracy, I would reccommend the Red Dot Sight (RDS), or Grip (If it's available) - Many guns which have larger, or more obstructive iron sights can cause a great impedement to aim. Grip lowers the overall recoil of a gun.
A class with Scavenger + Sleight of Hand may be a helping hand if you have serious problems with accuracy.
I would not reccommend using a shotgun, but if you do, you should probably use the SPAS-12 - I find this the most effective. The only maps that are really worth using a shotty on (other than the DLC, as I don't have them) are probably Nuketown, WMD, and maybe Launch. You should use it with a Supressor, and throw on Ghost, and play stealthy - Shotgunning on Black Ops is not like other CoD games, where you can just get kills by rushing the enemy - if you attampt that on BO, you'll just get demolished by SMGs, because they just don't have the same raw power as those on the other CoD games.
I think they are the best tips I can give.