For guns firing solid shot, don't ever go higher than a light rifle. Useless against armored stuff, could be a threat to unarmored units, and a hilarious surprise against BA. For stuff with explosive shells, I'd go with the medium rifle.
Basically, the weaker, the better. If a mech-only player looks at your record sheet and grudgingly admits you might be a threat, you did it wrong and need to downgrade.