Reality: US M16A4 with RCO and an M7 bayonet, its useful and reliable, don't fire while having an affixed bayonet.
Fiction: DC17M Commando Carbine, with replaceable attachments for Sniper fire, Anti-armor grenades, or regular fire mode. (A high energy plasma-rifle perfect for a less civilized time).
DC-15S Self-recharging Pistol, used by Clone Commandos. (Advantage is it never runs out of ammo, disadvantage being it may need to recharge every few seconds of fire.)
If we're talking in terms of design, Dreamcatcher from Final Fantasy: Crystal Chronicles is usually cited as the first weapon I would get a replica of if I were to ever get one.
Ever After from Kingdom Hearts III is a pretty cool one, too.
If we're talking in terms of efficiency, Evergreen from FFCC is very nice. It doesn't have the most smacking power, but it's got support abilities! :D