It's hard to say which had the most views, especially when taking into account illegally obtained stuff, online views, etc. Though the box office records usually give an approximate measure. Not perfect, but like I said, still approximate.
Pokemon: The First Movie brought in more money than any Pokemon film to date, about $163,644,662, so my best guess is that it also had the most people watching, which would make sense since it was during the biggest rush of Pokemon's popularity. The only movie to come close, Pokemon 2000 (note, still during the wave of popularity), is still trailing by about thirty million.