For me it's Gen 1. The number of times I've played through Gen 1 has broken my pair of nostalgia goggles most people see these games through. For starters the game is riddled with bugs some of which can be potentially game breaking and not fun like Missingno. Not to mention that I found Kanto and some of it's Pokemon extremely bland uninteresting. At this point there are much better Pokemon games that deserve more praise but it seems Gen 1 will always be regarded as the "best" .