>>58272160A few additions to this post:
Emerald mixes the plots of the two Hoenn games that came before it, so instead of going against Team Magma or Team Aqua, you instead go against both teams as you are caught in the middle of their fights as a third unaffiliated party. In Ruby, you go against Team Magma and in Sapphire you go against Team Aqua.
Emerald also locks you out on contests until you get to Lilycove City and in Ruby/Sapphire the progression suggests that you can participate in contests as soon as you hit Verdanturf Town, which is about 2 badges in as opposed to 6 badges in Emerald.
This is all to say that I don't think you should make it your goal to get the trainer card stars, you should instead play the game as you see fit while checking out these optional goals and see what you like as there are a bunch of releases that go in different directions when it comes to side content things.
Ultimately every entry point into the series is a good place to start and there are very few games that I would not recommend someone start out on, but I say Ruby or Sapphire because in addition to the standard games, they are in my opinion the most well-balanced in the series for having a few non-essential things to try in tandem with support for cross-console compatibility (Pokemon Colosseum/Gale of Darkness) while also having a fairly unique mix of things to try in those base games.
It's also very easy to say "you should start out on [insert overrated version] here!" but a lot of the times these versions recommended are recommended as such for their post-game content or alters to the base versions, to which if you are not going to touch that much or recognize anyways feels lazy to recommend otherwise.
