depending on what you're really into
I can say the motorola droid should be the bare minimum
that's what I have
it's fast enough to be very usable although not quite as silky smooth as the iphone
but it plays video at phone maximum resolution beautifully, navigation works as well as a standalone device...better than some
browsing feels like a pc limited to 3g speeds. even loads scirra chat and runs it correctly.
gaming is smooth, except for the camera. that's as slow as molasses, and unless you can temporarily stop the earth from spinning, you'll get motion blur. also, you press the shoot button, and then thirty minutes later it takes the picture
aside from the camera i never feel like it's running slowly basically, everything is responsive enough that i don't have to think about the response speed, so..
everything above that in processor speed should be very nice indeed
someone had an htc hero at a party recently and the screen was stupendously beautiful and huge
haven't seen the wildfire in person yet
if you're on a budget motorola droid is a great phone
if you're not, well, take your pick
I don't remember which, but some of the new ones can be used as a wifi hotspot for several devices at once without being unlocked. the droid can't
so if you might need that, keep that in mind as well
as a side note. all the hubbub about the crappy keyboard on the droid is blown way out of proportion. it's not the best keyboard i've ever seen or used, but it works, I can type on it just fine, wish the rows were staggered instead of lined up, but it's not the horrific mess people make it out to be