9 is hardest, I find
2,6, 12 are all very popular and very challenging.
15 is highly recommended to frustrated people

10 is awesome and very hard and generally overlooked.
11,13 and 14 are quite enjoyable too.
18 has a great "catchy" theme, but what comes before is a bit... boring (IMO)
you may also want to look at the Spanish Rhapsody, there is some genius use of upper register in there.
Basically most of the HRs are interesting and challenging.