I think the R390 and 787B are harder to master than the GT-ONE, but once you do, I believe the R390 is quicker than the other two on most circuits when fully maxed out, and once mastered the 787B is the easiest to drive IMO. I think the 787B is the fastest car in it's initial spec, but doesn't get the outrageous turbo upgrades the other two Le Mans cars get (after all, the 787B isn't a Turbo car!).
So, speed with initial spec: 787B > GT-ONE > R390
Tuned to the max: R390 > GT-ONE > 787B