Love this thread! I have played lights, heavies, and assaults primarily, and there are some mechs that are just a blast to play, whether or not you always "pwn face" with them. I have Mastered Jenners, Stalkers and Jagermechs, and each of them have some real gems.
I think the Jagers ended up being my favorite, combining decent speed and armor with heavy firepower. Played intelligently, they can handle a wide variety of situations, from lights to assaults. They also seemed to be so much easier to Master, as they racked up kills and damage so easily.
JagerPanzer is a Jager I arrived at mostly by accident. I had tried various all energy builds and so on, but I thought I would give the dual Gauss a shot. OMG is it fun! When I first started playing, I thought Gauss was the lamest weapon in existence, with its odd charge up behavior and tendency to explode in your face and kill you if hit. Something changed though - I got the hang of it, knowing when to charge up, peek, shoot, and step back. I've also gotten good/lucky enough to nail quick moving lights when they charge or run away. Playing as a light forever, I know what to look for in their movement patterns, and it has certainly helped in playing against them. This is definitely a mech to play with the Advanced Zoom module.
SturmPanzer is such a blast to play. You hang out just behind the firing line, waiting for the inevitable light to try a run through your Assaults, or to flank. You target them, get a lock, and start your shotgun + SSRM barrage. Most go down fast, and the wise ones turn and run. What is surprising is that it still packs a decent punch versus larger mechs as well. Just don't go toe to toe, play it smart. Very good at getting kills once the brawl has started and enemy mechs have some stripped armor locations. I love this mech.
Wraith is my brawler Stalker. I know those words shouldn't ever go together under conventional wisdom, but damn if I don't enjoy the hell out of this build. I have also traded in the pair of LPL for three LL (one in each arm, one in CT) and that works great as well. Super fun build. Absolutely great for making C-bills as well.
I am in the process of working through the various Pheonix mechs I acquired, but as of yet haven't found any that match the fun of some of these older mechs. I think the Shadowhawk might in some configurations (flying AC20 FTW!) but I don't have enough drops under my belt to speak intelligently to that point as of yet.