For me - I always usually choose the cadenza that I think suits the era of the original composition. Since Mozart didn't write a cadenza for the No. 20 (as far as I know) I'd go for something that sounds like a classical era cadenza. This way it doesn't seem out of place from the whole work.
The Beethoven is rather close. I do like it and it doesn't stray too far from the style of Mozart.
For me - the Alkan cadenza sounds way too harmonically out of place for Mozart - he goes through chromatic rising harmonies, and changes the style to a very sweet romantic sounds passage and it really doesn't suit the work when you listen to it as a whole. And at the end - it sounds like Alkan is just taking the piss... (suggesting - look what I can write).
The Hummel does use techniques similar to some of the Cadenzas that Mozart did write in other concertos (like his No. 23 in A Major). Yes, it does try and get a bit more technical than Mozart really allowed in the No. 20, but the harmony doesn't sound that obscure compared to the whole.
The Klansky doesn't sound too bad. It sounds quite playful and I think Mozart was definitely the jokester. It sounds quite early Romantic, sort of something Beethoven would write, so I consider it just as good as his. I must admit though - he does look like a bit of an oddball when he plays the piano - Maybe that's where Lang Lang got his inspiration from

As for Naoumoff's - No. Didn't like it - to me, completely out of style with the harmonies for a Mozart cadenza.
Anyway, that's just my take. Haven't heard them all - but it was an interesting question. Magic_hands - this is quite an intellectual topic you started. I like it.