To me, its a no-brainer. I've played 'em all, and OOTP wins hands-down - the best historical capability, lefthanded batters (
)I started on Mogul too, but I'm not sure how its any better a starter game than OOTP4 or 5 say (the only point of confusion is the Waiver system, and that can be turned off).
That said, their are plenty of other people who have a different opinion. The two guys above prefer Mogul, and there's some that like Puresim. As the chap above said, play the Demos. I never looked back after finding OOTP. I don't play any other games any more.