OK, I'll give you a strictly personal opinion. I can play Clair de Lune, Reflets and FI, I gave a shot to the other ones several times and they're probably within my reach with lots of work. So, on a grade of 1 to 100 I'd say :
Clair de Lune is 40
FI is 55
Reflets is 60
The three Liszts are IMO at or above 80. Rhaps 2 seems easiest, let's say 80, then I'd say Mephisto 85 and La Campanella 90, though Mephisto is much longer and touches many more aspects of technique. But I've always felt that the Mephisto laid itself quite well under the fingers, contrarily to La Campanella (or even the Rhapsodie)
Anyway thats just me,
hope this helps.
Btw, I'm new to the group, Hello everybody!