It's the most divisive generation.
There's two dominant types of long time fans of the Pokemon community, 1. the ones that play for the new content in each game, or 2. ones that play to vicariously relive their experience playing Red for the first time every single time they boot up a new Pokemon title. Both of these are extreme simplifications, but 90% are going to fall into one of these categories.
The first type will typically have Gen 5 in their top 3, fairly often being #1 or #2. If I had to summarize why, it's a soft reboot of the series, and brings a lot new to the table. There's tons of new Pokemon, cutscenes as a series first, overhauled the battle sprites, and was the first game to make experimental tweaks to things like the E4, evil team, etc.
The second type will typically have Gen 5 in their bottom 3, most often their least favorite. In this case, you basically liked Pokemon exactly how it was, and you really didn't want a ton of crazy changes. The entire game is built from the ground up to prevent players from just "running around and catching their favorites" most notably because BW1 locks them away until post game. Most of these people didn't even play BW2 after being burned by BW1, a game that to them simply wasn't worth playing.