Black PS2 - The worst out of the three and also the cheapest, has problems reading discs at later points in it's lifespan.
Satin Silver PS2 - The enhanced version of the above, comes with a built in IR sensor for the PS2 DVD remote and has no disc read errors. Is completley soundproofed which is great. Cheaper now that the PSTwo has been released.
PSTwo (A.K.A Slimline PS2) - Smaller, quieter, has built in internet function (!) The best and most expensive choice.
All three can play DVD's, have 2 USB ports, connect to the internet, and can support a 20gb Hard Drive.
So depending on your budget, i'd say get the PSTwo, but if you want to spend less then the Satin Silver is great too.