Do you have frequent access to a wireless network when away from home (i.e. college campus, work, McDonald's, etc.)? Then I would suggest to go with the iPod Touch and iGet Mobile (or something similar) for complete access to your entire iTunes Library back home, more apps and games than other iPods, a bigger screen for movies, and just the dang cool factor.
Are you in need of a phone as well and are OK with committing $1500 or so over the next 2 years on the phone and required service? Then the iPhone (and iGet Mobile) would be even better (no wireless network needed, just a signal from AT&T).
Do you just have some phobia about being more than a few inches away from an intangible digitized music collection? Than a suitably sized iPod Classic is probably the answer. The same goes if neither of the above apply to you. Unless:
Do you run/jog? Then a Nano is probably the answer thanks to it's no-moving-parts flash memory and Nike+, which is particularly cool, in my opinion (and may soon come to the Touch). It's not as powerful as some of the portable GPS solutions out there, but it's significantly cheaper and still useful enough as a training tool. Keep in mind that the Touch and iPhone are also flash-based, so Nike+ is the only real bonus (for now) going this route since both of those have bigger capacities and the ability to connect back home...
The Shuffle, of course, is out of the question...
My music collection is about 18GB (plus another 30GB or so between the wife and kids), another 4-5GB of photos at any given time, plus 60GB and growing daily worth of movies on my server. Obviously, taking it all with me is not an option... I went with the Nano, even with the "monetary leverage" to buy whatever I wanted. Next time it will probably be the Touch.