Anyone who is not listing Campus as last is wrong, IMO. That set is re-orderable. Personal desire does not = it's worth.
Here's how I see it:
1) 7-11 (rare outside of Japan, and in high demand)
2) Golden set (requires getting 3 ores per piece, which are rare, and then getting any one pieces is random)
3) Mermaid set (it's easy to get any one piece, but takes a lot of time and effort to get every single piece)
4) 1 million bells
5) beans (these are more rare, but I haven't seen these sell for over a million, to be honest)
6) Campus set
That's the international listing. If I was doing this by Japan demand, it would probably be...
1) Golden
2) Mermaid
3) 7-11 (this has already been distributed in Japan 3 times, so in Japan, it's not really that rare)
4) 1 million bells
5) Campus
6) beans (because it's natively in the Japanese game)