You'll find quite a few example layouts here:
http://info.melodeon.net/info/layouts/clubOne of them is my favourite, can you guess which?
There's no right or wrong here - it all depends on what you want to do, and how much effort you want to put into your playing. I think melodeon layout design should start on the chords, and then figure out the treble half row to go with it. The two manufacturer systems here, castagnari and saltarelle, take very different approaches and have different advantages. Castagnari's has more distinct chords, and is hence more flexible in what you can play - it's just not very flexible in *how* you play it, since many chords are one direction only, and force the treble to be played in a certain way as a result. Saltarelle's allows more flexibility in giving more chords in both directions (notably G and A), at the expense of the extra chords, so it is more flexible in how you play, but not in *what* you play. Does that kind of make sense?
For personal taste on a 'conventional' 12 bass layout I'd go for Saltarelle's (I actually prefer unisonoric basses, but that's a whole other can of worms), but Saltarelle's 21+5 layout on the treble needs tweaking to fit - why they don't do it like this initially, I really don't know - swap the direction of the F/Eb buttons (all of them!) to Eb/F, and change the C/F accidental to something more useful like C/D. The reason is simple - F needs to have an F chord to match with, and that's on the pull, and Eb is most commonly played (by me, anyway) with a B major chord (i.e. B minor without the thirds).
Cheers,
Clive