I've never completed the Pokédex in any games. I tried in Blue, but since I had nobody to trade with or the patience to evolve all my Pokémon, I never got around to finishing it. I made a semi-serious attempt at completing the National Dex in X via the GTS, but I ended up giving in after a few months as I didn't mind much enjoyment in it.
Never. I could have in Pokémon Fire Red, but I don't trade, and I honestly don't care about filling dexes as long as I completed everything there is to do in the game.