Day to day use: Silenced 9mm weapons. Plentiful ammo, lots of room for mods, stays sneaky with a silencer unlike some of the larger calibers.
9mm turrets in labs: .45 does just fine. I use a modded USP because it’s freaking amazing and ammo is everywhere.
Milspec turrets: M14EBR (again, modded, pretty much anything I can slap on these bad boys)
Spitters: I have a laser in my finger. It gets the job done.
Big, scary, nasty ass robots: That’s why there are anti-tank weapons in the game.
Mycus: Inter-Continental Ballistic Missiles. And even those don’t always work. Mycus OP, nerf Mycus.
But pretty much the only character that has access to these guns also punches enemies into chunks so…
Realistically, anything my chars can lay their grubby little fingers on. I hoard things, in the knowledge that I will one day need them. This leads to accumulating thousands of rounds of caseless ammo, hundreds of mininukes, dozens of LAWs and anti-tank rounds for other weapons, all while my char is running around plinking at zeds with a Cx4 Storm.