This will be sort of a run-off between the last 4 smartphones I've owned. Let's go backwards and start with the conclusion:
Conclusion: I prefer the Nexus One to any of the other phones. It's fast, it's pretty, has a lovely screen, sounds good through headphones, feels good in my hand, and most importantly, it can easily be operated with one hand (but maybe not for the reason you think if you know me ;) ) The trackball, larger than on the G1, is useful and not too inaccurate.
Evidently the Google way is to emphasize one-handed operation, and I find that this is really important to me. The phone can sit in my pocket and I can control the volume with the external volume rocker switch without even pulling it out, and I can pause, play, skip a song or answer the phone with the buttons on the headphones. I can do most operations on it with one hand and the trackball. All of these are totally important on my bicycle, or even if I'm doing some serious walking.
Also, although the Nexus One doesn't come out of the box ready for hacking, it's pretty easy to set it up for that and maintain it that way. At least Google doesn't try very hard to prevent you from doing it.
Sounds quality is important to me as a musician; the Nexus One isn't the very best sounding device, but it's within shouting distance of the Nokia. It plays Ogg files out of the box, and I expect it to play FLAC files any day now.
I love the Apple iPhone 3G. I especially like the well-stocked app store, but I have various problems with it, mostly having to do with Apple's attitude towards people that want to open up the phone a bit, documented here.
I won't go back to the iPhone unless this changes. Ironically, I like MacOSX on computers best because it so succesfully straddles the proprietary and open source worlds. Too bad they are so very afraid of openness on the phone.
The T-Mobile G1 was never meant to be my permanent phone, it was basically a placeholder until I could pick up a Nokia N900 something better in the Android world. It is ... OK. It has a truly terrible sound chip and it really sounds bad with headphones, and it doesn't have a headphone jack, you have to use an adapter with its USB connector to use headphones, which is clumsy and unpleasant at best. The trackball was useful, but so tiny and inaccurate it was difficult to use. On the other hand, it was fairly easy to "root" (the equivalent of jailbreaking an iPhone, which is a real pain to do and maintain) and made me familiar with the Android OS, which I got to like a lot. I recently dropped it in my jacuzzi, which gave me a good excuse to get something better.
Then there's the unique Nokia N900. Lots of fun, but it's simple not made to be operated one-handed at all, other than in phone mode. I kept sitting down at a desk and playing with it, and having fun, but then thinking that if was going to be sitting down with a computer I'd much rather do it with my laptop. The N900 seems heavy and bulky, and although it definitely has the best web browser of any of these phones (it even has Flash!), I found myself wishing I could access my Google online apps in mobile mode; a 3.7 inch screen is very small for normal web pages with my elderly eyes.
BTW, I was not going to use anything other than a GSM-type phone so I can take it on my irregular international excursions, so the Droid was not an option.
I gave my iPhone to my wife, who loves it an has no ambitions to hack it. The G1 was not that expensive. I certainly can't afford both the Nokia N900 and the Nexus One at once, but the price of the unlocked phones comes out to be less than you'd pay for them if you bought them subsidized by T-Mobile (and you can't do that with the Nokia anyway) over the span of a two-year contract. I have 30 days to return the Nexus One if I don't like it, and I got the N900 for a special price from Dell which is no longer available; I should be able to sell it for most of what I paid for it.