For Core: Doesn't really matter, if you have common sense, or a personal preference as to what your trying to accomplish in the game mode you'll know what to pick and what to do with them.
For HardCore: Since bullets hurt more in this (alot more), I would suggest taking the stealthy route...
Game Mode Used For: Team Tactical
Primary weapon: Thompson (Round Drum)
Secondary weapon: Magnum
Primary Grenade: Frag
Secondary Grenade: Tabun Gas or Flare
Perk 1: Bandolier
Perk 2: Stopping Power
Perk 3: Steady aim
Vehicle Perk: Ord. Training
This is my favorite Class for medium to small...