Which is the best gun to use?

The AWP (sniper rifle) upgraded to mythic quality is my choice for overall best gun. It's targeting range is beyond the normal field of view, normal shots penetrate through enemies, and with a rate of fire upgrade attached it fires quite fast. Fully upgraded you'll be mowing down enemies you can't even see yet, which is amazing for keeping your distance and kiting bosses as well. I haven't found a reason to use any other weapon really.

I use the mythic firebow. It's quick, fires 5 fire arrows at a time and sets everything on fire. When you get the fire rate chips in game, those five arrows become 8, 12 or loads. Doesn't last long but does the job.

Mythic Electric for me.