The ubiquitous assault rifle with combination grenade launcher. An oldie and a goldie, this one weapon design fulfills all your basic needs, from close range offense, to medium range massacre. True enough, you can't attack anything from long distance with this, but in lieu of that weakness, this combination negates a sniper rifles' two others: limited ammo capacity and slow firing rate. Anyone that's ever played an FPS knows that sniper rifles are not medium to close range weapons, and attempting to apply them as such usually ends in bloody death.
The assault rifle/grenade launcher is a basic weapon with nearly unlimited uses. Need to cover your retreat? Send up a wall of explosive fire before you, and make like a tree. Need to clear a hallway, or some charging enemies, and can't risk the moment of vulnerability hand tossing a grenade would cause, or that your enemies might run past it or dive for cover? Impact explosives again become your best friend. Need just basic fire support to clear some enemies out, a way to carve a path for yourself and not blow your fragile skeleton into powder? Enter the usually decent magazine size and speedy firing rate.
A combination of this type is very meat and potatoes, but also highly balanced. You can go from close to medium, from high powered offense to defense, to defeating cover and/or creating your own, from groupie killer to giant killer, all within the same gun, and usually all within the same two buttons or keys.
This style of weapon is far from being the most spectacular or flashy. It's certainly not as impressive as, say, the Spartan Laser or the Gravity Gun, but it's got broader applications for offense than either. I could be dropped into just about any FPS with just this type of gun, and probably play at least most of it, if not the entire thing, without needing anything else.
Look at it this way: Try to imagine a situation where this is the only type of gun in your inventory, and you're going "What a useless piece of shit, I might as well put it to my own head."
I'm sure there ARE such situations, but the harder you have to think to legitimately come up with one, the better it speaks for the weapon.
Best examples include: The assault rifle from Half-Life and Half-Life 2, and the SuperDragon from Perfect Dark.
