Nokia would be my top choice, no doubt about it. The Series60 platform is just incredible if you're a geek.
Plus, the general consensus amongst most professional reviewers is that Nokias almost always tend to have the best RF, which is a big criterion when selecting a phone, after all.
I do have my eye on the Audiovox SMT5600 at the moment though (an updated version of the Orange SPV C500, for you UK folks). It's the world's smallest Windows Mobile powered smartphone. Comes with Outlook and WMP10, both of which I would love to have.
The higher-end PDA-style Sony Ericssons are cool (the P800/P900 etc), but the lower and mid-range line is too boring. Although they're very well-designed phones, I just don't like the OS they use. It's not extensible enough to keep me entertained for more than a few days at best.
Motorola - they're generally nice and professional (v300, v600), but again, too boring.
Samsung - their phones in Korea are something to drool over (they just announced a 5 megapixel camera phone there), but based on the phones they've released here in the U.S., they would probably be at the very bottom of my list.
Sendo X and iMate - never seen them, never used them, so I wouldn't know.