What's the overall feeling in these cities in east/southeast Asia?

It's my husband and I's 5th anniversary this year and we're planning on a vacation. Since we go around the US and western/central Europe a lot, we're thinking about going to somewhere different. We both love big cities so we're in between Hong Kong, Tokyo, a trip to both Kuala Lumpur and Singapore, Beijing, or Hanoi. What are your experiences in these cities if you've been, and what's the best place for a week long trip?