Looking back on the last three years, I have to go for Armin van Buuren. Above & Beyond produce nice tunes, too, but to be honest, they first have to be remixed by other people to become really amazing tracks.
I like what Tiesto has done in the last year. He's experimenting with other electornic music genres, and every track he releases is an absolute club burner. Ferry Corsten did some great stuff in 2006, I think, with his "electro-trance" album. It really pushed some boundaries. And looking back on System F or Gouryella, he was once one of the major forces in Trance. Nowadays, well, not so much...
And Paul van Dyk – I don't know. Most of the tracks he produces sound stale to me, like he doesn't have any ideas at all. An exception from the last year might be his collaboration with Johnny McDaid, "Home". But otherwise, neither his productions nor his DJ sets convince me.
But my favorite producer is actually Leon Bolier.