Hi Heat, I'm a trance musician, and I'll take a stab at this.
There isn't one synth a guy like Van Buuren uses. He has racks and racks of hardware synths/effects, and will pick up any soft synth that sounds...well...good.
Generic answer, I know, but his list, as well as Van Dyk's, Oakenfold's, Tiesto's, Corsten's, it's all tens and tens of thousands tied up.
I remember he loved Spectrasonics stuff, especially Atmosphere, Stylus RMX, and Omnisphere of course. He also loves the Access Virus keyboard line. I owned a Virus kC myself, which is actually in the hands of an electronic musician guy at Sweetwater.
I'm picking out a lot of different synth sounds in there. The lead sounds like like a thick, enveloped saw lead with a realllllllly nice low-pass filter.