As much fun as dropping a CP from across a courtyard and behind cover with a cinder block was in Half-Life 2, I'm not going with the gravity gun.
I'm tempted to guy with Shephard's sidearm from Half-Life: Opposing Force. The very first gun you get, and it can drop half the things you see in a single shot. It's basically the magnum, but with enough ammunition that you can use it more than five times in the whole game.
Then there's the crossbow from Deus Ex. But while it's an incredibly cool stealth weapon, any of the more combat-oriented characters will quickly find that it is useful only for dropping GMOs, and is more-or-less useless against humans.
Soooo... I'm-a go with Opposing Force's alien grenade launcher. It's an iguana-looking creature creature that just perches on your arm. Give it a squeeze, and it vomits out a corrosive glob of explodey green stuff. Plus, you can pet it in your idle animation. And that is awesome.