The mediums have it I think.
They tend to be a good combination between survivability and fire power.
Most mediums can take a hit or two, but can't withstand any concentrated targeting, so it encourages mobility.
Mechs like the Phoenix Hawk have a decent throw weight, are quite mobile, and reasonably well protected. You can add the Centurion, Enforcer, Griffin, Shadowhawk and Wolverine in there too.
Give everyone pretty decent gunnery and piloting skills, nothing is more demoralising than lining your opponent up and realising you need 10s at medium range. Make it possible to hit each other, and the game wont last an eternity.