Firstly, and obviously, each generation can trade with itself, like Red and Blue or FireRed and Sapphire.
Secondly, the 1st gen and 2nd gen games can trade with each other through the time machine in the Pokemon Centers in 2nd gen games. However, only gen 1 Pokemon with gen 1 moves can be traded, which means no sweet scent, no waterfall, etc.
Thirdly, the GameCube RPG games can trade with any 3rd gen game, but the traded Pokemon can't be Shadow Pokemon unless they've been purified.
Finally, actual "trade" between 3rd and 4th gen games is impossible, but it is possible to migrate 6 Pokemon at a time from a 3rd gen cartridge game to the Pal Park of a 4th gen game. (These Pokemon can never go back to the original game.)
Thanks, but I guess that means I'm out of luck. I want high quality Pokémon in R/B/Y, but the mechanics are so odd that I have no idea how I could tell if a Pokémon was good or not before training it to level 100 and having it participate in an ungodly amount of fights. I can't really get the Dodrio Gameboy in Pokémon Stadium without said Pokémon so that won't be able to help me. I was hoping that by some strange method I could either trade for or Pokésav a good generation 1 Pokémon with the best (generation 1) moves and stats in generation 4 and get it back to R/B/Y. But I guess it wouldn't work thanks to IVs, EVs, natures and such.








