I deliberately omitted a "they all have their uses" option because while it is entirely true, it's also a cop out. If you absolutely had to say one weight class was better than the others, which would you pick?
I suppose the problem is the fact that there are 2 game types. Lights can win a game by cap in either, but it is much harder in Assault. I do like the Heavies. I remember when people discovered you could run 2 gauss on a k2. My table was flipped that day, and I ran out a purchased a k2 with my hard ground c-bils as soon as I could.
Heavies are the best by far. For me, at least. Catapults and Cataphracts cinch it for me, but then look at your Jagers as well. The Dragons can out medium a medium (I think) and the jury is still out on the Quickdraw. The Orion will also be divine. Heavies. No doubt about it.