Always seems to sneak up on me, so I hate the Puma D's guts. I specifically target any Puma D's I see. I love shooting them in their stupid faces when they delude themself that they can blast through my LBX oro's front armour in the time it takes my weapons to recycle twice...
I'd vote the Bushy E though for it's brilliant mix of allround firepower (seems weaker armoured though).
I also kind of like the Thor A? (the one with the UAC20, SSRM, ERLBL, and flamers). It's got a weapon for every occasion, and if it had a bit more armour and the ability to look down it would be quite a potent variant.