Beethoven: many, but for some reason my favourite is probably #30....28, 29, and 32 are my other favourites
Haydn: many, can't decide, but perhaps #45
C.P.E. Bach: the father of the sonate, perhaps...I have liked many that I have heard, but for some odd reason 55/4 sticks out, perhaps because I have a thing for midi's and it is one of only a few of his works in that medium
Boulez: #3, even though only two movements have been published, always really gets me. For some reason the latter two movements of his #2 don't appeal to me (perhaps it's the performance I have), or else I would say it
Schönberg never wrote a work which he titled a sonate, but I consider his 3 pieces op.11 to be close enough. One of my very favourite works, I love to follow one of Beethoven's great sonaten with it (in my listening practices; I can only dream of playing such pieces)