Two:
Second place, a .50BMG. Just obnoxious - especially when I didn't initially realize the muzzle brake is deflecting rearward --- right towards where I was standing. Once I moved it wasn't quite as bad.
First place, a Marlin .45-70 Guide Gun, ported barrel. The shooter had his muzzle under the overhead sheet-metal awning and the blast was truly painful.
This Marlin got first place because his muzzle (and ports) were behind and under the overhang, thus deflecting the sound back down onto us on the firing line. I could feel the concussion in my head and after the first ten rounds, I had a headache. Five rounds more, I packed up and left. That is the first and only time a gun has driven me off the range. The guy with the .50 had his muzzle out in front of the roof overhang which made a big difference.
Q