I think everyone has a sentimental attachment to their first Pokémon game (Blue for me) or perhaps the first one that they put time into and completed, but from the RBY generation I must admit that Yellow is the best game due to the additional features (notably a Pikachu in tow!)
However the "best classic game" has to go to Gold (or perhaps Silver because of Lugia). They remain the best Pokémon games and two of the best games I've ever played.
Crystal is also excellent but I wasnt much of a fan of the Eusine plot or Unown messages having already played Gold. If the mobile phone system was implemented outside of Japan then Crystal would have been amazing.
Gold and Silver have great characters, mind-blowing ideas, a good story, flawless gameplay, excellent map design, and after all that you get to revisit the Kanto region that you know and love.
Incredible.