I absolutely love reading these threads!
Best:
New York (so much so me and DP are eloping there next year)! We also loved San Fran, Chicago and D.C. in the states.
So many countries in Asia... Singapore, Thailand (Krabi and Chiang Mai specifically, not so much Bangkok!), Cambodia, Hong Kong. Amazing people, incredible food.
We visited New Zealand earlier this year and the scenery was breathtakingly beautiful. The South Island is truly stunning.
In Europe we’ve loved Santorini and Rome. Some parts of Spain are beautiful too.
Worst:
(I’ve never been anywhere I hated, but there were a few places I wouldn’t be in a rush to return to)
Beijing in China felt very unfriendly. We made the effort to learn some basic Chinese phrases, but it was so difficult to get by or get any help at all as a westerner.
I’ve been to Sydney twice and just don’t “get” it. The harbour area is nice but I don’t understand why people rave about it.
Also not a fan of Cyprus. I’ve never found anywhere as charmless, but maybe that’s because I’ve only been to Paphos and Ayia Napa 
Iceland and Croatia are high up on our list of next places to visit, so I’m glad they’re being mentioned on this thread!