I'm still figuring out the gun thing, but so far I'm having the best luck with the type 25. I prefer AR's for their situational flexibility. The gun isn't so important as the purpose and perks of the following builds I like using.
Objective: This class uses the Type 25 AR w/ reflex sights and the quickdraw handle and a Tac .45 as the secondary, no attachments. That will remain consistent in the next two classes. Perk 1 is flak jacket so its more difficult to explode me while taking an objective. Perk 2 is fast hands so I can throw grenades back, use lethals/tactyicals faster while taking an objective. Perk 3 is dexterity just because I like to aim faster. I tried tactical mask as my third perk, but its no longer as necessary while taking an objective as it was in Blops 1, as nova gas has been removed, and there seems to be little use of stun grenades and flashbangs in this one. My lethal is a grenade so I can explode other people taking an objective and tactical is a smoke bomb so I'm not as exposed while taking the objective.
Roaming: Perk 1 and perk 2 are Flak Jacket and fast hands, but they aren't essential. I just thought as typing this it might not be a bad idea to make this class a ghost class w/ blind eye and cold blooded. Perk 3 is engineer so I can find nifty little pieces of enemy equipment around corners. My lethal is a plain old boring grenade. And the purpose of this build is the engineer perk coupled w/ the two EMP grenades, which destroy every shock thing, claymore, bouncing betty, stationary gun or piece of equipment or killstreak the enemy throws at my team.
Axe: This one has no purpose but playing w/ my favorite weapon, the battle axe or from blops 1, the tomahawk. The important parts to this build are scavenger as perk 2 and danger close as a wild card. This way I can carry 2 axes and replenish them when I get kills. More axes = more axe kills. Axes, fyi, are excellent anti camping revenge weapons. If somebody is lying prone, there is a good chance they are camping out there and they aren't moving. Which means you just need to pop out around the corner and throw it at where you already know they are still laying. Its even easier if they hiding in a corner, because if you miss, it'll bounce off the wall and still get them. :)
Anyway, those are three classes I find to be very useful.
I also saw one guy playing w/ six perks built for speed and stealth and a knife. He killed me a bunch.