Do you mean anime series or games? I love the Diamond and Pearl generation for the anime and for the games I love the 4th generation games.

of course of serena

Black 2 was the best Pokemon game that I ever played.

X and Y, all the way! The only Pokémon core series games I dislike are Red and Blue Versions (Yellow is a little OK). Black 2 and White 2, and Black and White are great, too.

black white- worst series ever
Xy best

If we're talking about the anime, this is how I would rank it:

#1. Diamond and Pearl
#2. Sun and Moon
#3. Advanced Generation
#4. X&Y(Z)
#5. Original series (+Johto)
#6. Best Wishes

As for the mainline games:

#1. Generation 4
#2. Generation 7
#3. Generation 5
#4. Generation 3
#5. Generation 2
#6. Generation 1
#7. Generation 6