OK, I had to reinstall MW4. Just remembered that one of my faves was the Shadowcat because its so damn useful. That left arm is a 3 slot omni-point. You can put all kinds of cool shit there.
Also my fave throughout much of the early game was the Uziel. Each arm is a 3 slot beam hardpoint, and twin 2 point ballistic sections in the chest, not to mention some missiles in the center. Overall, a real winner, especially if you prefer beam and gun over missiles, which I do.
Catapult was nicer than I remember. A trio of 2 point beam sections on its chests, in addition to the uber-missile packs. I have been giving it to one of my lance mates as a low-end sniper. Fill the beam points with Clan Large Lasers and then a buttload of LRM's.
For me: I use the Uziel but in an odd way. Instead of two PPC's (which are painfully slow) I filled the arms with Clan Med Pulse lasers and then a bunch of light AutoCannons in the chest. No missiles because of tonnange limits. Its super at close & medium and my mates can add fire support at long range.
As for Heavy, it has been and always will be the Nova. Just too much fun with an assload of large pulse lasers or several PPC's and some small lasers.
Assault: Daishi. Nothing comes close. The slight armor advantage of the Atlas is minor because its loadouts are always weak. Very few 3 point sections and they are mostly ballistic, which means you'll max out tonnage long before you fill all the slots. Wheras the Daishi is mostly Omni-slots, meaning you can put whatever you damn well please on it.