Nail knot is the easiest once you get above 200lbs I think.
I have successfully formed 3 turn (only 3) uni knots in really heavy mono and fluoro though, often with the aid of pliers. I remember managing this and winning a bet in the process with Jinkai 300lbs.
For GT fishing though, is it really necessary to go above 200lbs single strand? I use 170lbs as standard and sometimes venture up to 200, but no more.