Well, it all depends on what you mean exactly - the thread says "greatest forehand", the poll says "biggest forehand", they're not necessarily the same. In this second category, Gonzalez undoubtedly wins for sheer power, but it's no use if you can't get it in the court consistently.
I'd actually go for the Vowel Man himself, El Aynaoui; especially after watching him eliminate Hewitt at the AO, you've got to be impressed by how easy and natural a shot it is. He doesn't have a huge takeback like Ferrero and Fernando, he takes a compact swing at the ball, unleashes a low and menacing growl of exertion and pounds that forehand all over the court.
Notice also that all those players with really big, notable forehands rarely have a decent backhand; only Agassi and Safin are evenly balanced on both sides in terms of power, not really having a single big shot off the ground, although I think they are both more comfortable on the backhand side.
I'd say Ferrero probably has the best backhand in conjunction with a really big forehand, and I say this only after seeing how well it held up in those punishing rallies with Agassi at the Masters Cup. Of the others, the most effective is again El Aynaoui's, particularly when he plays the one-handed slice, but even then all his winning backhands are more a matter of luck than design.