For the Commando I think the AUG-HBAR is the best gun, it has a mix of good firepower and excellent accuracy, for a machine gun. For the Assassin I think the AWP or Dragunov. AWP for horrible aimers and Dragunov for those who can do headshots well, combined with the laser, it can kill pretty quick.
Why do people like katanas so much? Assassins with melee weapons is the same as a tank with a shotgun. But the difference is that tanks have more hp and longer range. So in my opinion, melee weapons are useless with assassins, they should be with tanks. Also I have 3 new favorite weapons: AWP (instant kill) Minigun (lots of ammo and good dmg too) Cyclone (it's a secondary weapon, but just as good as a primary)