This is the BEST thing you'll see all day.

14-year-old New York Mets superfan Braeden got hooked up with a once in a lifetime experience at spring training in Port St. Lucie, FL this week ... meeting all the players, hanging in the clubhouse and on the field!

Not only did Braeden -- who is battling cystic fibrosis -- hang with guys like Michael Conforto, Jacob deGrom, Jeff McNeil and Tim Tebow ... he also got to take the lineup card to home plate (in full uniform) before the game against the Cardinals on Sunday.