Aurel... I'm a total cell phone geek. Here's what I would say...
VZW - BB Storm 2, Droid Incredible, or wait on the forthcoming BB Slider, that will have a physical keyboard
Sprint - HTC EVO... possibly a better device than the new iPhone
AT&T - obviously the iPhone.
I've been on the verge on getting an iPhone for years. However, when all is said and done, AT&T is just awful. It's not even a close second to VZW where we're at. And all rumors point to an VZW iPhone in the second half of the year. I'd be shocked if it doesn't happen by September.