That sure feels good to get an individual mention right of the bat in a thread by the renowned Clay Death.
I don't like to comment on players I haven't seen play very often, so my discussion looks at more recent players.
After a bit of thought, I'm going to put Nadal's forehand as number one. It's well known that Nadal's forehand generates more topspin than any other ground stroke ever in the game. This spin enables him to take tremendous rips at the ball and still have considerable margin. This margin means that he rarely misses and this gives him even more confidence in pressure situations. The topspin also makes it so difficult to volley and consistently attack, plus it allows incredible angles. Put this on clay and the vast majority of opponents simply can't handle it. Add leftyness and amazing speed (running around backhands) and you have one of the most effective weapons in tennis, ever.
Nadal's forehand may be best overall, but it certainly isn't the best in all conditions. For all round effectiveness, look no further than Federer. He also generates a lot of topspin, but not as much as Nadal. His forehand is generally hit with more power though. Give Nadal the right forehand and he'll ware you down pretty quickly, but Federer will usually hit a winner quicker. It was scary how dominant Federer's forehand was in his prime. Just slightly more errors are enough to push him back from number one in my opinion.
Often a good way to find players with an amazing stroke is to look at the rest of their game. Moya and Gonzalez didn't have good backhands but were both very good players. Hence their forehands must have been excellent. Moya ran around his forehand heaps and had every right to given that it was one of the best shots in tennis. Kind of like Nadal's, yet flatter and less consistent (and a righty). Gonzalez's forehand was extremely powerful and when he was in the zone, it was virtually unstoppable.
Special mentions to Agassi who was amazing at taking the ball on the rise (on both wings including the forehand) and Sampras with his incredible running forehands. I don't feel brave enough to rank players beyond the top two, but Nadal and Federer have a clear lead (among relatively recent players).