Depends massively on what you like.
Personally, I've been to cities and also to beach places and also to slighly more unusual places.
To be honest, the cities are not that different from each other. Shops, hotels, cars, traffic. But for an interesting time, maybe SHanghai or Chengdu or even Moscow.
If you want 'an experience', then Las Vegas, but maybe plan on staying in that centre of Mammon for only a day or two and using it as a base for the Grand Canyon/Death Valley/areas.
For me, the two places in the world I remember best are up beyond the Arctic Circle in Ivalo, Finland. And visiting the desert outside of Dubai. Not too much to do there, except burn up the sand on off-roaders, but nevertheless, quite an experience.
Similarly, southern Tunisia to see the Sahara...
If it has to be a city, then the two I love most are London and Shanghai. Both have a massive sense of energy; restaurants of every taste and budget and a huge range of things to do, as well as effective public transport.