Puma: ERPPCs for missions; CLBX20, four ERMeds for Solaris(I regularly bag 4-6 kills and usually don't take too much damage in return. EDIT: To be fair, of course, 2-3 of them are often a hair away from death by the time I actually run into them, so that's just cleanup or simple kill-stealing, but the 200k bonus per kill doesn't care who did all the work.

). I think only the Wolf's Dragoons and Kell Hounds get them in the shop, though.
Also, I might try out the Jenner IIC if you like the Cougar, but find it a little too twitchy.
Stormcr..er, Ryoken: 4ERLL(the Medium Jungle matches are on a water map, so just keep your distance and keep your feet wet); 2 CLBX10, two Heavy Medium Lasers. I love the super-fast torso.
Black Knight: loads of choices, though 4 ISPPCs isn't bad to play with at all(if you don't like the heat, there's always 2ISGauss, 1ISPPC, same damage, no heatsinks needed). It's also available pretty early in the Campaign.
Timber W...Mad Cat: Again lots of choices, from 6ERLL, to various iterations of ERPPC/Light PPC/Gauss/CUAC2s or 5s/LRMs.
Orion IIC: Generally not fast, but very durable, good torso twist range, and slightly above average turning for a high-end heavy. It can pack a punch and carry more armor than other 75 tonners(plus AIs and people alike tend to have a hard time figuring out where the Center Torso actually is).
I'm finding the more I play with Heavies, the less I like most non-100(well, make that 95-100) ton Assaults(if you're willing to take a little extra heat, many heavies can offer comparable power and much better agility/speed). A jumping 3ERPPC, 2CGauss Behemoth can be fun, though the Behemoth II w/2ERPPCs and Railgun is a lot easier to build/manage heatwise.