My worst experience (so far) has been Las Vegas. Only one bad experience though. We took a very short cab ride (we should have walked, but we didn't know exactly where the restaurant was located), the meter said $5 at the end, the cabbie turned the meter off and demanded $10 in a threatening manner.

My best experience has been mainland China (specifically Beijing and Shanghai). Drivers were efficient, courteous, used the meter every time without being asked, and refused tips, even if they drove a bit crazy sometimes. Taxis were everywhere, cheap and very convenient. We were very obviously tourists and the only Chinese words I knew were "please" and "thank you." Used taxis a lot there during a 7 night stay, and we always used official taxis. Maybe I just got lucky though?