I think the humble Flounder has gotta get my vote though Dabs come a close second...But as i think about it most species can be ridiculously easy to catch in some circumstances..LSDs , School Bass and Smuts all seem to have suicidal tendencies at some time..
The easiest fish to catch in WSF waters was the Bettlefish. It couldn't resist baits like Quicksilvers, Cape Verde lures, or Fuerteventura jigs. Unfortunately global warming round the gills got the Bettlefish a bit steamed up, and it departed. It may return in camouflage, however.