That all depends.
Solo: Robots & Mercs are sturdy. Have good CC, Damage, & Toughness. Especially with new Drone and Upgrade to Serum to effect all Mercs.
Duo+ Thugs. Thugs will buff Thugs. So if you get say 8 Thug MMs together, all Enforcers will buff all Thugs. So that's 16 Stacks of Enforcer Buffs & 8 Stacks of MM Leadership buffs Power Pool.
Reason I say Robots/Mercs. They are not hard to learn, they are not melee(well Medic is stupid, likes to rifle butt everything). As for Secondaries, Forcefield, Thermal, Dark Miasma, & Trick Arrow are good options for those 4.
If your experience with MMs in general. Then look into Undead, Beasts, Ninjas. They are very focus fire heavy. Mean, pick a target, hit your attack button, rinse repeat. (Though Undead with right buffs / debuffs can get away with more skirmish fighting)
As for Thug's solo. They are the probably most complicated, when it comes to making sure shit works right. You got your Mini Brute(Bruiser). You got Enforcer with their AoE & Buffs, You got 2 Gangers & Firebug. Problem here, Firebug aka Pyro. He draws so much aggro and can't take the hits. So you'll be babying his ass a lot. The set is a mix of Melee & Ranged.