As an lvl 49 commando,
Top slot: sniper (three-shot rail gun magenta maliwan that does enormous damage)
Left slot: torgue spinigun (shoots 10 rounds per second with long range and explosive damage...great for badass goliaths and spinner mechs)
Right slot: vladof pistol (10 rounds per second up to 15 with Sheriff's Badge)
Bottom slot: rocket launcher
I don't use shotguns or smgs. I use double up with longbow (+110% turret health). Haven't gone to Gemini because all the gun damage perks I would lose--about 40%-50% gun damage.