Define best?
I am a total Windows Mobile convert now, could not live without it. But most people will hate it, and with good reason in most cases. Not to mention how bulky they are. You really need to be a bit of a masochistic geek to enjoy it.
It all depends on what
you want from a phone. Tell us what features you want - Music? Camera? Ultra compact? Looks? Games? Internet/Email? The abilities to run lots of excellent feature rich third party applications, such as Sat Nav, etc? You will almost certainly have to trade off a few of those features for others, there is no perfect phone. It's always a trade off, even when money is no object.
I always recommend people pop over to
Mobile Review. It's the best mobile review site on the net if you ask me.