Light: Wolfhound, a energy boat that looks good and has decent speed and moves. I usually go with a LL or PPC in the arm and couple of small or medium lasers as back up. Oh and I also like the Osiris with it's great speed, jj's and great short range firepower.
Medium: Bushwacker due to my personal config of 1 LBX-20 AC, 1 LL, 2 SSRM6'S and I sometimes switch the SSRM'S for LRM's for missions that require fire support.
Heavy: Thanatos, drop the JJ's and weapons and add 1LL, 1Gauss, 1LBX10, 2 ML, 1SSRM or LRM. Jack of all trades with little heat problems. I also like a RAC varient Argus that I came up with.
Assault: I want to like the Templar but the huge side torso's usually get smashed in early so I will go with the Sunder. Great mech with alot of firepower, armor and also with enough speed to actually get to an objective in decent time. 1PPC, 1GAUSS, 2LL, 2ML, 1 SSRM6. All around firepower. [drool]