I've been rockin the Thanatos prime lately, and have found success, I have yet to stay in the field long enough to use the two free tons of ammo on the MRM20 though, spamming it at everything including Ba and Aeros, I always need to repair first.
On the flip side, the Warhammer Prime has lost its shine with me, I can't seem to pilot anything slower than 86kph without getting stomped anymore...

The Mauler Prime is awesome but endlessly frustrating because of the depressingly bad damage the AC2s do to ground targets.
I had a UAC scat come up to be and he nearly killed me because the hours and hours of AC2 fire did shit to his UAC arm, I finally damaged it enough with my lasers to scare him off though. If you stay back it can do nice damage if you fire the LRMs at any narcs and spam the UAC2 on everything you see.
I'm looking into trying the Hgauss Thanatos and trying out the Hollander again.
I keep telling myself I will use the Atlas Prime,A,B, but I always think the better of it.