The Rival 9 is just consistent. However I do enjoy some of the other SMG's.
The swarm is good.
I also love the AMR9, for some reason I do alright with it.
I've just finished my camo grind.
But I usually have a few favourite classes sitting about.
BAS-B, Rival 9, SVA.
There are others, but those are the main 3 imo.