I have an iPhone 4 and an HTC Surround, and, although it's a little different situation since I can switch back and forth between the two whenever I wish, I've found that, since getting the surround, I havn't even turned the iPhone on.
WP7 is, in a word, amazing. I've used android, and obviously I've used iOS. I've even played around with WebOS. WP7 is the prettiest and smoothest. It's still very young, but it more than makes up for its shortcomings with what I feel is by far the best UI design. I still love my iPhone, but I think that, for the time being at least, I'm sticking with WP7.
So of those 3 choices, I'd go with the samsung focus. Personally, I want nothing to do with Android unless it's a handset that uses stock android and has top specs, so I don't ever have to worry about being stuck on older OS versions. And neither of the 2 you mentioned offer that.