Of the current era Safin and Nalbandian, except for Nadal on clay. They arent as consistent as Federer, Nadal, or Djokovic by a long ways, but when they are on they beat anyone (again except for Nadal on clay), including Federer, and have proven it many times.
McEnroe would be way up there. 1984 McEnroe was as good as anyone in history probably as far as playing level. Sampras on his best game on a medium to fast court was nearly unstoppable. Becker too on fire on a carpet or grass court in his younger years was nearly untouchable. The 1996 YEC between Sampras and Becker might be the best match ever.
Rios and Nastase would be two others. Both underachievers but both played at a godly level at their best. Laver when he was on was almost untouchable, and before him Hoad and Gonzales would fit that.