None, they're all c*rap. I was given MS2K as a present, IT IS NOT REALISTIC GUYS! If you just want to pass time, and not aiming to be serious, fine, it's probably worth it.
If you're serious, career-wise, don't go anywhere near the thing. Spend money on proper flying lessons, or get to a place which offers the public sessions on the REAL sims. Even they are not perfect, but close, and uncomparable to PC games. I have "flown" the Trident and 777 sims (777 with BA), and yes, they are expensive (£200 the BARE MINIMUM I might add), but they are as close as you and me are going to get to flying heavy stuff for a long while yet.
In the UK, the BA sims, are found here http://www.virtualaviation.co.uk
And for those of you who know what I mean: no, I am not just quoting ppruners, it is my own honest opinion, which (I am happy) concurs with those who should know.