GTA Vice City is by far my favourite as well. Everything about it was awesome and what I want out of a game like GTA. It was an over the top tribute to the 1980s TV shows, movies and in general. It had the Lawyer from 'Carlito's Way' the House from 'Scarface' the lead from 'Goodfellas', the Ferrari from Miami Vice/Magnum PI... it was just perfect. You could live in this larger than life world, buying up property, piloting helicopter bombs, shooting everyone and getting into massive carchases.
And to the people who said GTA2 sucked, YOU SUCK. GTA2 fucking rocked. It was pretty difficult at times, but the city was huge, it had lots of missions everywhere, you could put MACHINE GUNS ON YOUR CARS, the gangs were funny and cool (a team of mad scientists and escaped mental patients for example).
As for the sense of humour, I barely even remember the fart button, but it was just a fun thing to put in to replace the horn for when you were on foot. Apart from that , saving your game at a church (with a big 'Jesus Saves!' over the top) was funny, and many of the missions were really awesome or funny as well.