Alright back on-topic, the best class is not always the BEST for every map, there is a best class for each map, depending wheather it is a big map or a small one, also the players on the lobby also count, if you happen to go in a lobby with a bunch of pros from clans and stuff, it doesnt really matter the class, but how fast you react and how good you are, but if you happen to land in a lobby with average players and a couple of noobs, then you are good to go! Here are my favourite (best) classes I have used
For small maps-
P90 + Silencer (Rarely anyone uses it, but its really good)
Spas12 + any sight you want (it pwns if you know how to use it)
Stun or Flash
Sleight of Hand Pro
Stopping Power (Necessary with P90)
Steady Aim (This is a MUST with the p90, you will see itll be so EASY to pwn noobs with hip fire hehehe)
OR as knifing class for small maps
UMP45 + any sight you want, recommend extended mags if you have it
Magnum .44 + tactical knife (I dont use tactical knife, I think its gay, but it helps)
Throwing knife (Stun them and run in on a rampage!, fast and humiliating for the enemy)
Commando pro (no its not noob if you use it in small maps)
FOR BIG MAPS
ACR+any sight, for dark maps or if you want to camp, I recommend thermal, ITS BEAST WITH IT.
SPAS12 with any sight, holographic is good.
Semtex and flashbangs, claymores if you wanna camp.
Scavenger or Sleight of hand pro
FOR BIG MAPS AND KNIFING
UMP45 + silencer
Magnum .44 + tactical knife
semtex or throwing knife + stuns
Lightweight pro or Cold Blooded pro
Quick tip for knifers when in big maps such as derail, afghan and etc. try to take the biggest way to your objective, the way you believe no one ever takes, or shouldnt take, and sneak on in their spawn, once youre there... prepare for ultimate ownage! I had amazing scores with this tactic in derail!
for average sized maps, such as skidrows and stuff
AK47/SCAR H/M4A1/TAR21 + any sight
SPAS12 or Magnum .44, its good for switching fast if you run out of bullets
semtex and stuns
Sleight of hand pro
Stopping power pro
Ninja pro or Steady aim pro if you like hip fire drop shots close in on the target