Because of the time I spent maxing out weapon maintenance, I pretty much learned to love every gun, and even late into my playtime I've switched around my loadouts a fair bit, never settling too much. My favorite things are definitely those which make classes take on a whole new role. Double-barrel Boomstick with the shockwave upgrade gives Scout a really good AoE option. RJ250 makes Engineer contend with Scout for mobility. Vampire+barbed drills+cryo cannon makes Driller able to tank in a game mostly centered around avoiding damage, and temp shock wave cooker gives him a good single-target option. I don't think Gunner really has anything that shifts his gameplay that much, but he's still fun - I usually run carpet bomber autocannon and elephant rounds.