The most reliable weapon is still the UT Gatling series from the Fencer. Theres nothing like firing bullets for 2 straight minutes and shredding an entire horde, may it flying or ground based, in the process.
The most OP weapons in the game are DLC and high non-DLC Phalanx. It kills an Erginus in two charges on Inferno.
For Fencer it's DLC shotgun plus Jackhammer + dash-hop. Almost like a cheat, but sooo much fun.
Same can be said about Buster Shot for Ranger which should be on the number spot for all "best shotgun in a videogame" discussions.
Air Raider is OP overall if you bring the right stuff to the missions. Don't sleep on Beam Shot (single shot laser)which is fantastic against frogs, because it one-shots them and gives you this fun risk-reward mechanic, where you can shoot again if you hit but have to wait when you miss, so you can create some sort of combo-shots.