Now I'm at the stage of trying to pick a Hero 'Mech reward. The real challenge begins
What are your priorities looking like?
For me, it's canonicity; stuff like MAD-BH2, CN9-YLW, and VND-1SIB tend to grab my attention. Only the last one doesn't actually carry that designation in canon – the actual
St. Ives Blue is a modified VND-1R, though it was eventually standardized within the Compact as the VND-1SIC – while MWO's CN9-YLW has some fudging to account for the fact neither melee nor rear-facing weapons exist in this game.

For you, maybe performance is what you're looking for... I heard good things about
Mad Cat MK II "Deathstrike" and
Dire Wolf "Ultraviolet".
Locust "Pirate's Bane" and
Piranha "Cipher" are a pretty frequent sight for some reason. If you prefer heavies, maybe check out the above MAD-BH2 or the
Hellbringer "Virago".
Otherwise, just pick what speaks to you the most.