Gen 2 -- Who doesn't love the huge after game and having multiple regions to explore. This was more an expansion and correction of Gen 1 along with some nice new twists. Ho-oh and Lugia were the first instance of legendaries being a focal points of the story. The final climactic battle at Mount Silver is something I'll never forget.
Gen 4 -- This legendary trio was my favorite, and Sinnoh was my personal favorite Region. Not to mention Cynthia makes for a great champion. They also had the leaders involved in the story quite a bit. Finally, this had some of my favorite pokemon designs.
Gen 5 -- I liked the pokemon designs, hate me I know. This gen gets a bad wrap because of Garbodor and Vanniluxe alone. The region was very fun to explore, N had a great story to him. Black 2 and White 2 were just amazing sequels and Neo Plasma shoved back at old plasma who was sort of dorky.
Gen 7 -- Very humorous and a good rebound from the dreadful story that was told in Gen 6. Great characters and development, decent rivals. Gamefreak listened to the criticisms of XY, and that's what I like to hear.
Gen 1 -- It was fun and innovative for it's time.
Gen 3 -- This gen just wasn't my cup of tea as a whole. I liked the concept and designs of the Legendary trio as well as the two different evil teams, but everything else really was disappointing to me. Well maybe the inclusion of abilities was neat.
Gen 6 -- I could write a book on why I loathed XY. This game was a culmination of all things I dislike in story telling and gameplay (single player wise). The ORAS remakes were also incredibly boring. Megas were nice for making previously poor pokemon viable, but the story was just dreadful and gameplay mindnumbingly easy.