At first i was good with storm bolter because it was op, then it got nerfed, then i was good with nurgle ground assault because it was op,then it got nerfed, then i was good with power fist JA because that slamming action was op, then it got nerfed. JK i never "got gud" with anything because im not a role player. I just abuse what ever is the most viable weapon, currently mostly bolter and swooping hawk.
Stalker bolter (way back when it wasn't a magnum). Then plasma rifle/pistol. Then mercury pattern bolt pistol.
Power fists before they got nerfed. I'd run high stamina builds, sacrificing all of my survivability for an extremely aggressive playstyle that would leave me dead in a ditch against 2+ targets. Now? I can't even drop shields...
The first iteration of the reaper launcher. Imagine an autocannon that fired as fast as you clicked but with explosive rounds. It was godlike. Also rokkit launcha. Don't tell anyone, but it's still pretty good.