It's probably a tie between FR/LG & Let's Go
While the Sevii Islands were a nice addition that foreshadowed the events of HG/SS, I just remember how annoying FR/LG were due to the Kanto dex lock and the help screen making the game feel too hand-holdey (even moreso than 3D pokemon games have been). I'm not as mad about the dex limitations in future games, but at the time it was so annoying since I either wanted to try using Johto or Hoenn pokemon on my Kanto journey or I had to deal with all of the times pokemon like Golbat were forced to stop evolving. Plus, the cutscenes for stuff like items & TMs got old really fast.
I'm a little more lenient towards Let's Go since it features Green in the postgame, it has a good balance between being modern and faithful to Yellow, the art style's a great homage to the original Red & Green illustrations, & it's the first game to implement Go transfers. However, the games are still a lukewarm experience overall because of the catching mechanics causing me to spend so much money on pokeballs, the EXP system due to said catching mechanics makes this feels more like a grind to play than any other pokemon game, & Trace being the worst friendly rival in the series. This is actually the first pokemon game I haven't finished, & I'm not sure if I ever will.