My top 5 in no particular order:
PIRATES' BANE
Or as I like to call her, how to be irritating 101, will she pull top damage, top kills? not a chance in hell, but will she be a P.I.T.A. to the OPFOR? Hell yes! and in the case of some mechs, a literal P.I.T.A.
YEN-LO-WANG
The amount of times, that this mech has let me snatch victory from defeat is amazing, also helps that she is in olive drab, to blend in noob CN9's helps, a lot.
DRG-1C
I finally got her back, after take a clan brake, and all I can say about her, is I missed this mech, nothing impressive, but she can hold her own, and be such a P.I.T.A., helps that I've gotten better at twisting, and aiming since I first ran this thing when I got started with MWO
AWS-8Q
Yes it's bad, but man, when you take down a Dire Wolf with this thing does it make it worth it!
BLR-3S
Yes, she is a little undergunned, but she's so fast, and doesn't fall over dead from ST loss, and that is enough to stun some people long enough for me to get in that last hit.