M16A4 & M4A1
See CoD 4 entry.
G18
MW2 brought machine pistols, this was the coolest. A lot of people complained about recoil, I personally found few faults with the weapon and it was nearly on par with a lot of the primary weapons. A sniper's best sidearm.
Model 1887
I don't care if they were OP originally in MW2, they were cool. The old design, the cool swing round reload, the shooting across a map with a shotgun. Even after they were modified, they retained the first two parts and still remained a useful secondary weapon. (These comments refer to dual wield, single wielding these should be illegal)
Scar-H
This was a great assault rifle to start with especially since MW2 was less generous than CoD4 in its first level gifts. A very good weapon to use while waiting to unlock the ACR. Admittedly it made the M4A1 very pointless, but I still used the M4A1 now and again for the CoD 4 feeling.
ACR
Just the best CoD weapon ever. Almost everyone used it. Very precise weapon while still being reasonably powerful. I imagine my K/D drastically altered from Level 48 onwards each prestige.
Intervention
Since MW2 was my favourite CoD game and I love using Bolt actions, it is only fair to include it. I got to Red Tiger on every prestige with it thanks to it being available from the beginning. The design of the weapon is very good compared to other bolt-actions too.
Tactical Nuke
It might be a killstreak, but it's still a weapon. Now I'm not CoD master so I only ever got three of these, but all of them were a great gaming achievement for me. Plus my username is Nuke so I might aswell give the inspiration for it some recognition.