Usually the sequels of a game doesn't end too well, but BW2 is a different story. I love how the game starts you off as a new character but with familiar characters like NPCs, gym leaders, and antagonists as well as Team Plasma. It also often mentions the player from BW1 which I found interesting. These small references from BW1, but new changes to the Unova region really made the gameplay fun for me. The storyline also did not disappoint me, since it really connected well from the postgame of BW1 to the "2 years later" timeline of BW2.
I remember playing BW1 around 10 years ago and I recently caught interest in playing it again, only to discover that there was a sequel to it which I loved playing. The fact that it made me think about the 5th gen it probably meant that this generation was really memorable. This, I think was the most successful series from the entire franchise, in my opinion.