Reason 4 for me, it's like a bottomless pit, your only limitation being running out of CPU lol!
I like it because it's really intuitive, quite a novel way to produce tracks, and being able to wire your virtual rack up however you like is awesome.
With that said though, I only have experience with Fruity Loops besides Reason 4, and I thought that was excellent too, just a little on the unreliable side at times.