My most used weapons are:
M4 - I just love this gun, good rate of fire, its not quite as powerful as I would like but if you are quick enough with it is still deadly (got my K/D to 1.93). I use it with the 2 X Claymore, Stopping Power, and Martyrdom perks.
Barrett .50Cal - This sniper isn't for everyone, easily the most powerful sniper is a one-hit-kill with a shot anywhere except the legs... maybe. It has a big recoil but for me that is ok as long as I can kill one enemy, steady it and hit another without out alot of frustrating recoil. I use it with the Brandolier, Stopping Power, and Iron Lungs perk.
M60 - Just recently made this a new class, I never used it before but if my M4 ran out of ammo and I saw one of these bad boys I would surely pick it up. Once you get the grip and or sights for this weapon I find it pretty accurate. But still not used as much as my M4 or Barrett.
P90 - I used this for a week and got tired of it, the great rate of fire is good but I didn't find it powerful enough even with Stopping Power, in the time that I used it I picked up alot of kills and it makes an appearance every now and then, a decent SMG. I used it with the Brandolier, Double-Tap, and Martyrdom perks.
Like my fellow "Veterans" I have noticed alot of lower ranked players recently, I am guessing because of Christmas, there has been a boom in lower levels. I am loving it, say I don't want to big of a challenge I will go to Domination or TMD and find myself surrounded by lvl1-10 players making it really easy to rack up kills.
Just finished playing today for a while and had some great games, a domination at Countdown going 64-4-10, Domination Overgrown going 30-3 (sniping) and TDM Crossfire going 40-15. I love this game.