I wear an O'niell Hammer that is a 3/2. That's about the thickest (warmest) I've seen in terms or heater shirts. The only place I could find that carried it was a surf shop in Cali.
the 2mm jet pilot and oneill ones with glideskin are the warmest. glideskin helps so much because it is windresistant. wind cuts through standard neoprene
The new Jet Pilot F35 is the warmest most comfortable one I've worn.. .. it has the 'sharkskin' outside and the 'firefur' lining inside and it doesn't feel restrictive at all.. also looks pretty sick with the rasta 80s print... see a pic at http://www.tcwake.com/prod-Jet_Pilot_F35_Flight_Top-5496.aspx