Well, the GBA ones come in sets of 3, with a pair initially then a followup later on (like Red and Blue then yellow or Gold and Silver, then Crystal). Red and Blue started off in Kanto, with 150 pokemon to collect, however they each had several exclusive Pokemon you would have to do a cross-color trade for. Yellow was brought out later, and allowed had enhanced graphics and Gameboy color support. Later came Gold and Silver, which were GBC and took place in a new region, and had 100 new critters. Crystal came out later to complete that trio, but the only difference between the two was the option of playing as a female. Flash forward and Ruby and Sapphire came out, and they had an almost totally new lineup. Then was Firered and Leafgreen, which were nicely done remakes of Red and Blue, but had a bunch of new features, and can battle/trade between Ruby and Sapphire.
Then there are the console versions, the only one of note i want to talk about is Colosseum, which is VERY different from any of the GBA games with 2 on 2 battles exclusively and the brand new concept of stealing shadow pokemon, and can also trade/battle with the GBA versions.
So, buy any of them you want. They're all good stuff.
