Favorite without a doubt is the Timberwolf B variant, Very nice direct, accurate damage at ranges up to 850m, with optimum engagement distance at 600m. As much as people say ERMED's arent upto much, personally I disagree, 3 of them, with the refire rate they have, add a decent amount of damage onto a target at a good range when combined with the rest of the B's armament. With accurate fire this mech can take down anything quickly, and its very good for fire support into brawls and in general support of your allies, because of its reliance on direct fire weapons.
Close second comes the shadowcat, any variant, I love them all. The mech is fast, has an amasing turn rate, has good quality jump jets, and has some awesome loadouts. Main gripe is the B variant is far too expensive considering how subpar LBX's are currently.
Favorite light is probably the cougar C variant, with 2 ERLarges, 2 LRM10s and a Narc beacon. combined with C3 and JJ's, this mech is good at finding the enemy, passing the info onto friendlys, and setting them up as good lrm targets. extra ton for narc ammo tho pls devs =)
And my favorite starting mech has to be the Uller A, what more do I need to say apart from gauss and ecm?