Speaking of multikills..a few weeks ago, I was playing recon on the attacking team on Port Valdez. At the second stage, we got the Alpha Mcom easily with the tank, but the area near the hill on the right was getting 'nade spammed to hell from the defenders crowding at the crates. I sneaked up from the left near the ruins of Alpha and called in a mortar strike at the base of the hill. Holy crap...
7 kills from a single mortar strike 
My team simply rolled in, cleaned up the stragglers and blew up Bravo.
Just a few days ago, I had the best killstreak so far. Jumped into a tank at the first stage of Port Valdez (attacking) as gunner. The driver knew what he was doing, and his squadmate trailed behind, promptly repairing the tank as necessary. We reached the final stage of Port Valdez..up the hill; the team got Bravo, but couldn't get Alpha. In the process I got
25 kills in a row as gunner; the driver got 28 before somebody C4ed the tank. Each of us ended up with a final score of 26/2 and 29/1
I have become quite proficient with the following loadout.
Assault: AN-94 + nade launcher + RDS + magnum ammo
Engineer: UMP-45 + RPG + extra rockets + explosive damage (while attacking)
Engineer: UMP-45 + RPG + extra ammo + magnum (defense)
Recon: VSS + extra ammo + magnum + mortar (sometimes C4 on Panama canal)
EDIT: forgot to add.. the M93R (? the last sidearm in the list) is always my backup weapon. Excellent in CQB, especially for shooting enemies in the back.
I simply suck with the long range sniper rifles.
I rarely play medic these days.
For short-medium range combat, the UMP45 is probably the best weapon in the game, especially with magnum ammo. It's silent, accurate and deadly. On foliage-heavy maps like valparaiso and laguna presa, playing as a russian engie is awesome...you blend in with the foliage and pick off your enemies silently. I've wiped out entire squads this way.
For medium-long range, especially defense, the motion mine + silenced VSS is awesome. It does tend to run out ammo quickly, but the stock sights are great, and if the squadmate drops ammo regularly to replenish the motion mines, the advantage from the increased situational awareness can lead to lots of easy kills, especially on foliage heavy maps. ofcourse, they're invaluable for defending mcoms too.
For me, the key to playing well is stealth, stealth, stealth. Silent killing = win. Shooting them in the back = satisfying. If I go one on one deathmatch against LMGs or shotties, most likely I'll end up dead even if I get the first shot in and even if the enemy also goes down...so I kill them before they even see me, and then change my location.