I think you need to define 'screw gun' to get better options.
Some common things people call screw guns
Bosch Drywall Screwgun
Bosch Pocket Driver
Makita Hammer Drill/Driver
I could give other examples. If you narrow down the category or the purpose for the tool I can definitely provide some opinions. My overall favorite tool ever, for driving screws is the Makita Impact Driver. If I were doing drywall hanging all day I would use something like the Bosch. If i was doing just a few screws now and then I would use the pocket driver. If I wanted something multi-purpose, for drilling and driving screws I would use the Makita Hammer Drill/Driver.
Oh, yes, I own or have used many times every one of the above tools.
Some Drills/drivers compared here
Episode #9 - Drill It!