I'm very unfamiliar with the sonatas of Haydn. What are they like in comparison to those of Mozart of Beethoven?

Also,

Which would you recommend to start with?

Which are your favorites?

Which publisher do you recommend?

Which recordings do you recommend?

I'm not too familiar with Haydn sonatas, but pretty much if you choose one of the earlier ones in C major, D major, or G major, you're good. None of them are really too out of reach for anyone, but look over the scores first to see what you think. My favorite (and one you can play if you figure out which one I'm talking about) is in D major and starts with a C# grace note into a D, then I believe the D drops down an octave into D eighth notes-or something like that. It's one of the earlier ones, and I'm sorry I'd tell you more but I can't find the score...
The score, btw, is Schirmer and while many people are quick to stay away from the Schirmers, there really aren't too many things "wrong" with the Haydn sonatas, IMO, and it's good especially if you don't want to spend a lot of money. One thing you'll notice though is that when you're looking at different editions you're almost guranteed to find differences between, say, Peters and Schirmer, as far as ornamentation goes and, in some cases, the actual notes themselves! But get whatever ones you want to-obviously if you have the money go for Henle. But for me personally I'm not going to spend that kind of money on Haydn, just because I know I won't play it too much...
As for recordings, I guess it depends. I haven't heard a lot of them, but I'd recommend Gould for the sonatas he recorded. For the complete set, I have no idea...Gould didn't do one did he?

Best edition BY FAR is the Landon edition (Vienna Urtext.) It's expensive, though, so if you want something cheapter that's okay, just go Dover. I don't recommend Peters. Last time I looked, it had all kinds of weird editorial stuff in it.

The one Goldberg is suggesting - D Major (Hob. XVI:37) - is excellent. So are F Major (Hob. XVI:23), C Major (Hob. XVI:20), b minor (Hob. XVI:32), A Major (Hob. XVI:5), A Major (Hob. XVI:26), and e minor (Hob. XVI:34).

And many of the early ones are also good. There is an early G Major sonata and an early F Major sonata that I've also taught several times. (Forget the numbers, though...)

Also, you should be aware there are two numbering systems for Haydn sonatas. The numbers I listed above are the Hoboken numbers. These are probably the best known. However, the Landon numbers (which look like "L. 31") are gaining in popularity and are better. (They're in much better chronological order.) So when you're looking up the sonatas, make sure you know which numbering system is being used.

For recordings, I'm not aware of a good complete set (nor do I really think it would be worth owning.) There are some excellent Haydn CD's out there though:

Emmanuel Ax
Leif Ove Andsnes
Alfred Brendel
Andras Schiff

And on a personal note, thanks for bringing Haydn up. I LOVE the Haydn sonatas. I've played 8 or 9 or them and they're WONDERFUL pieces. (Personal faves are A Major, Hob. XVI:26 and A-flat Major, Hob. XVI:46)

Kreisler, is that G major by any chance the one marked "Allegro e innocente" for the first movement, and the second movement "Presto"? If I didn't mention it earlier, I had to play it for a competition...I started out being highly critical of it, but by the end of the month I was loving it and hating the Rachmaninoff piece I had to learn. Anyway, if it's not the same, it's still a good piece IMO. Anyway, I am not familiar with the Haydn to know the Hob. or L. numbers, and sadly the Schirmer doesn't list either one. It's very strange, but then I suppose that's what you get for a $10 book.