Whats the best year for F40?

IMVHO the 1992 is the culmination of five years of production where each car was essentially a building block in a brand new process.
They never changed the spec (except for the USA market specific issues - 90/91/92) but they simply got better at working with CF and putting the pieces together. If you look at a Euro 87 and a US 92 the differences in finish quality are obvious.
The 92's were, for the most part a little more durable, especially in areas such as the engine cover hinges and the quality of the seats and interior CF joints. The car still looks like a kit car in many respects, but it is the best of the Ferrari supercars for me.