The Ian Hobson recordings of the piano sonatas are indeed excellent. Stephen Hough is slightly better, but this is pretty much a general rule for everything Stephen Hough ever recorded. I don't think one can go wrong with either.
Piano Sonatas "7 through 9" were written between Op. 2/3 and Op. 13, I believe, whilst Hummel was a teenager. I wouldn't call them essential, though I wouldn't call Op. 2/3 or 38 essential either. If you like those two, you'll probably enjoy 7-9 just as much.
Of the piano sonatas I would say Op. 81 is not only the greatest, but probably Hummel's best work in general. Its popularity throughout the nineteenth century was well-deserved. I'd put Op. 13 in second place, but that's probably a somewhat heterodox opinion; that said, it's the work where Hummel's style crystallised (not to undergo any significant changes for the rest of his career) and has a certain youthful freshness and vitality that I don't find in some of the later works once that style had occasionally turned into formula (e.g. stretches of Op. 106, which I think is a more conventional second choice).
The fugal finale of Op. 106 is also a masterpiece; the fugal finale of Op. 20, not quite so much. Op. 20 I think could have been an impressive work but comes across as curiously underdeveloped—perhaps the most interesting concept of Hummel's sonatas but he ends up backing down somehow on its promises. I'm not sure how else to explain it.
The weakness of Hummel's sonatas and the thing that makes them hard to tell apart sometimes is of course that he wasn't good at coming up with memorable thematic ideas. At his best, he's aware of this, and uses various strategies to maintain interest, eg the brilliant "pre-Chopinesque" ornamentation of Op. 81/ii which successfully enlivens the rather generic shape of his melody, or replacing full-blown themes with a kind of cantus firmus that moves polyphonically through the texture, but very often he nonetheless can't do without those full-blown themes and they tend to be some combination of schmaltzy and generic. Only Op. 81 avoids having any such themes, and it is also the most fun to play, btw.
For early Romantic piano sonatas not by Schubert, Hummel doesn't approach the quality of Dussek's La retour à Paris or L'Invocation or Élégie harmonique, but his sonatas as a whole are better than the majority of Dussek's, and about on a level with those of Clementi, differing chiefly in that Clementi doesn't really seem aware of his difficulty in coming up with memorable thematic ideas.