I'm torn between Gen IV and Gen V at the moment, though I'm really starting to like Gen VII.
Gen IV was my first Gen, and it had so much side content, it's crazy. You have the Pokemon Contests, the Underground, Poffin Making, loads of optional caves to explore in D/P/Pl, and the Contests, Bug Catching tournament, The Kanto league and lots of other things in HGSS.
Gen V has the best story (I haven't finished Moon yet, so cannot compare the two, though the plot seems really good thus far.) and a bunch of really good characters. B2W2 also have a lot of side content- Pokestar Studios, Join Avenue.... Plus the Unova Pokemon are pretty great! :)