Recon (preferred class)
M98B with 12x scope and straight pull bolt, suppression perk, a G18, and either MAV or SOFLAM depending on the style game and map.
Assault (2nd fav)
M416 with either PKA-S HOLO or IRNV, foregrip, and extended barrel. Again a G18 for sidearm, either suppression or speed perk depending on map/mode. Oh, and my trusty M320!
A-91 with PKA-S or IRNV, foregrip, and laser sight (sometimes the G53 with the same parts), G18, and explosion perk so I have more ammo for RPG.
Support (only for small maps)
USAS-12 with IRNV and tac light, claymores, and either explosion or speed perk. Again a G18 as a sidearm is a must for me. I'm currently trying to unlock the MK3A1 to replace the USAS.
I enjoy using recon 90% of the time (depending on what the squad needs). I usually run around quite a bit, but always try to keep some distance between me and the targets, to easily pick off people with the 12x scope. I never sit still in one place, unless I am spotting people with the MAV. With the assault class, I do a lot of flanking. With the engineer class, I primarily use the RPG/SMAW to destroy vehicles/buildings/walls. With the support class, I run around like a chicken with its head cut off shooting at anything that moves, lol.