The top contenders turned out to be: 22g FX Hybrid, 17g NSA, 20g Howler, 25g JSB Knockout.
I'm running a 350mm barrel with the Valkyrie Shorok silencer & handguard.
Michael - thats very interesting performance. Did you do any tuning for each of the different pellets and slugs? My .22 350mm barrel was a little less accurate that I had hoped OOTB with pellets. I lapped it with JB Bore Paste and accuracy improved quite a bit. Same with the .22 250mm barrel that came with the gun.