It's difficult to say. Everyone needs something different according to own needs.
If you want to be visible, buy 7650 or 9210. Very visible, very expensive, and parts for it are very expensive. Many small stability problems, no GPRS (in 9210). Weak battery in 7650.
If you need phone for calls, no WAP, but requires extended pbk, buy 6210 or 6250 (second is for any weather and hard environment). Will be able to "repair" (flash) it - it repairs each phone in majority problems. These phones are very slow and has some other small problems.