Of this particular group? The iPhone 4. Antenna flaw aside, it's fast, has an amazingly sharp display, and has software which is considerably more accessible for the average person than the others.
I can't help but get the feeling that the Galaxy S will be a big gotcha for buyers. Looks good now, but Samsung isn't known for supporting a ozone for more than a few months past its release. I'm really not counting on Android 3.0 for that device. And while the custom UI isn't the worst I've seen, most give the nod to HTC's Sense UI.
One thing's for sure: the BlackBerry 9800 has an uphill battle. RIM has yet to show that it actually understands touchscreens, and that's what the 9800 is in a nutshell. OS 6 really doesn't change the paradigms behind the platform, either, so it's going to feel like a pretty version of the software BlackBerries have had for years. Could be RIM finally jumping the shark.