I've ticked off most already - I made it my sole objective once funds were available, and when I was well and truly independent with no ties to get out and see the world. Places like Hong Kong were so good that I actually plan on perhaps living out there for a stint and learning the language.
New York is my second most visited city besides London - Amsterdam is the first. New York has a way of reeling you in and making YOU it part of IT. It's also THE place to go if you're a bit of a procrastinator and want to get stuff done. It will indoctrinate you into believing that anything is achievable, and that YOU have the power to do it (I attribute this to the people - even kids as young as 8 are on the streets trying to make a living.) Also, Brooklyn is the best place you can be on July 4th.
Seoul is the REAL city that never sleeps, New York stays up till midnight, Seoul is as busy at 4am as it is at 4pm. It really is the ultimate 24 hour city. Nightclubs just don't shut, or stop, at all, ever. If you're under 30 Itaewon is a must. And I guess Shinjuku, Tokyo too (just don't talk to or make ANY eye contact with anyone who approaches you trying to make friends or invite you to 'a bar they know'.)
I guess now my focus is on places like Africa, Bangladesh, the outback, and, actually, quite a lot of Europe. I really haven't seen enough of Europe. Even as a resident of the UK,I haven't been any further north than Luton. I should fix that.