A few come to mind:
BLR-2Dr Warlord: It's a great TSM design, getting to the appropriate level easily and capable of utterly shredding stuff up close, then punching like gauss rifles. If you stay at range, you've still got the PPCs to worry about. All with 94% armor and a fusion engine.
Flamberge 2: I'm not counting the fact that it would lose nothing by being 85 tons because that would disqualify a lot of other designs as well. I'm chalking it up to a cost-saving measure. Other than that, this thing is just unfair.
JM6-D4 JagerMech: It isn't necessarily the perfect 'Mech or anything, but neither is there anything I'd change about it. 5/8 and it can fire light over standard PPCs from each arm at a run with no heat buildup? And a backup quartet of MLs? With maxed-out armor? Sure. Absolutely. I'll even give the XL a pass since it uses the tonnage so well.
FNR-5X Fafnir: Granted, if your goal is "two iHGRs" there's not a great deal of leeway anyway, but I wouldn't do anything different with the few remaining tons. Not to mention the fact that, with the SFE, it's actually quite inexpensive.
BLR-4S BattleMaster: Jack-of-all-trades, master of most. 4/6, tough to kill, and at least one gun for any occasion. I think the Jade Falcons only took Pandora because they knew what would happen if they didn't. Although, the DA map shows it back in Lyran hands, and since they upgraded the factory to make the BattleMaster C, they may yet regret that decision.
Turkina X: Experimental technology done right. Run away from it, because there is no range at which it cannot utterly mangle you.