There is an order that I follow confident, but there is probably a few subjective numbering.
(From the easiest to the hardest)
EASY
1. Sonata No.20 in G major - Leicthe
2. Sonata No.19 in G minor - Leichte
3. Sonata No.25 in G major - Cuckoo
MODERATE
4. Sonata No.9 in E major
5. Sonata No.10 in G major
6. Sonata No.1 in F minor - Little Appassionata
7. Sonata No.5 in C minor - Little Pathetique
8. Sonata No.8 in C minor - Pathetique
CHALLENGING
9. Sonata No.7 in D major
10. Sonata No.6 in F major
11. Sonata No.11 in D major - Pastorale
12. Sonata No.3 in C major - Little Waldstein
13. Sonata No.12 in Ab major - Funeral March
14. Sonata No.18 in Eb major - The Hunt
15. Sonata No.11 in Bb major
16. Sonata No.4 in Eb major - Grand
17. Sonata No.14 in C# minor - Moonlight
18. Sonata No.13 in Eb major - Quasi una Fantasia
DIFFICULT
19. Sonata No.17 in D minor - Tempest
20. Sonata No.2 in A major
21. Sonata No.22 in F major
22. Sonata No.24 in F# major - A Therese
23. Sonata No.27 in E minor
24. Sonata No.26 in Eb major - Les Adieux
25. Sonata No.16 in G major
TRANSCENDENTAL
26. Sonata No.21 in C major - Waldstein
27. Sonata No.22 in F minor - Appassionata
28. Sonata No.28 in A major
29. Sonata No.32 in C minor
30. Sonata No.31 in Ab major
31. Sonata No.30 in E major
32. Sonata No.29 in Bb major - Hammerklavier
I think that this ranking is very good, although I can point some exceptions...
I think that Op.7 (Grand Sonata) is very harder than that. Op.28 (Pastorale Sonata) is much harder too. I don't know if it's juts me, but I think that Op.2/1 (Little Appassionata Sonata) is harder than Op.10/1 (Little Pathetique Sonata). I also think that Op.90 isn't that hard. I think that Op.78 (A Therese Sonata) is a little easier than that, at least easier than Op.31/2 (Tempest Sonata). I know that Op.2/2 is hard, but I think that it's not harder than Op.27/2 (Moonlight). For the others, I guess it's appropriate. Of course, Op.49/2 (Leichte Sonata) is the easiest and Op.106 (Hammerklavier Sonata) is extremely difficult.
Of course, I may be saying wrong, after all, I don't really play the Sonatas (only a few movements of the easier ones), but after listening to them all, I thought that these "corrections" shouldn't be wrong.
At last, I think that all of Beethoven Sonatas are very hard, with a possible "exception" of Op.49 that are easier. I love them all, and if I had a wish come true, I have no doubt that I'd wish to play them all with "perfection". It's my dream as a piano player, and I hope it come true. After all, the 32 Sonatas are the New Testament of Classical Music. They are incredible, just awesome! =D
hope i helped