I think a version of R.I.C.O's mech is currently the strongest physical build.
There are probably very few players with 3 mythical plates, but the best version would probably use Nightmare or Grim Reaper and 3 plates.
A version with Annihilation is probably the single best build of any type for players who haven't found the right legendary items. With Avenger, Iron Boots and a lot of upgrading, you could be competitive in the top 25, or at least the top 50 without any legendary item luck.
In fact, yeah, Grim Reaper + 3 plates is probably best. 2300 HP, 460/190 Heat/Cooling, 524/196 Energy/Regen, or mess around with the modules for slightly different stats.