Nadal. It's still effective despite him not being in top form this year. He can play short especially on the defense if he's not playing well, which he did during a lot of the clay season and IW, but as soon as he can dictate it's very difficult to get away from his forehand. That hasn't really changed this season.
Djokovic has a solid forehand and has an awesome slap, but his preparation is probably just a tad too lengthy to really dominate consistently with it and the shot will often produce weaker shots when attacked. I don't think he'll really improve on that wing anymore, but he has come a long way already. It's about equally good as it has been the past few years.
Wawrinka's forehand is very explosive but it's obvious where the problem lies: it is lacking defensively and can be erratic. When he's at his best it's as dangerous as his backhand though, but if he's at his best all of his shots are nearly impossible to handle anyway.
Federer's forehand has at least progressed this year. It's taken him a while to find the rhythm on that wing with the new racquet but I think he has truly found it now. Especially against Djokovic in Shanghai and against Gasquet today it was as good as ever. Nevertheless, over the whole year, it has been flawed, but when it's on it's still too much to handle. Certainly better than it was in 2013, especially at the end of the season.