Well traditionally I've played with Heavy Mechs supported by a handful of lights and assaults. It gave me a moderately mobile force, with a good deal of firepower, while still giving me the niche units to perform other combat roles.
Lately hwoever, I've been developing a growing interest in combined arms. I blame video games ;D
As such, I've been playing largely with Heavy mechs as the "anvil," while using quick, hard-hitting vehicles to round up and flank enemy units, and VTOLs to harass them from the rear. Combined with BA/PBI forces to help deny planned attack routes, it's proven fairly successful.