Marauder, Locust, Warhammer.
The 3025 stock loadouts is part of what made the marauder and warhammer so flippin cool. They were rock solid designs, compared to other mechs of the time. Most mechs were poorly designed in 3025, with oversized engines, undersized engines, too much armor, too little armor, too much heat generation, too little heat generation, too much reliance on one type of weapon, or too many diverse weapons.. Etc etc... Marauder and Warhammer just landed squarely in that Goldylocks zone in all respects.
Locust I just liked because of it's central role in Decision at Thunder Rift, and because it looks sweet.
Ugh so hard to choose.
I'm going to say Marauder.