Osiris B: I just love the AC20 it can be used to rip any light/med to shreds and if you see a heavier mech coming over the hill you can just go passive and run away from there at 160 km/h. When you get the feel for the 20's bullet drop it can also be used as a Howitzer (Infinite range

). And the best thing of all: It is available from the start. However i fear that my darling will be seriously nerfed in .3

Cougar C: A jack of all trades that can dish out lots of firepower with its 2x ERLBLs and 2x Lrm 10s. And if you need even more fire you can just bring the rain by tagging the enemy mech with a narc, or just wait until your allies sees the enemy thanks to your C3.
Thanatos A: A heavy gauss on a heavy chassie, paired with DSSRM 6 and an array of medium lasers, that is all you are ever gonna need. Just remember: don't try to use it as a sniper, the Hgauss us awful at range. Fun fact: people always goes for the legs when they are up against this one, however i often kill more mechs while I'm lieing on the ground then when I'm standing up (sens-moral: Don't bail as soon as your mech looses a leg, try to aim at the enemies instead).