Original game - bad gameplay, funny story but damaged by its own structure, lack of coherent plot.
Mk2 - definitely better gameplay, improved graphics but inconsistent framerate, coherent and focused plot, a bit more serious (caused quite a bit of people to dislike it), extra non-canon ending that has a shock value.
V - Improved graphics but recycle of dungeons, improved systems with some underused ideas, crazy story that is more focused than the original game. In the official topic buried somewhere (no time to look now) there's a lengthy post done by myself that discusses the game in detail.