The closest I ever got was in my Blue game, with 149/151. The only two missing were Kangaskhan and Tauros, both I could get in my Silver game, but never got around to it, then I gave it to someone else for some reason. Don't know why. Maybe it was completed, maybe not. Don't know.

Another was Diamond where I got 459/484 (as I had Manaphy and Phione), but since I was missing games like Ruby, FireRed and Pearl, I couldn't complete the last of it and I couldn't do anything else, so I restarted.

But completing the Pokedex gives you a Trainer Star and a Diploma. It isn't worth but it's good for bragging rights. And you can say that you have, indeed, caught them all.

I'm also doing this in my White game, currently 460/649. It's still a work in progress, but I'm determined to do it, even if it will be just the one game. And I'm also trying to get one of each as well, just to add to the challenge.

But if you have all five games for Gen III, you can do it. I assume just getting the ones necessary, as there is no way in hell you are getting event legendaries in this day and age. Also, getting the Johto starters will be tough unless you have Colosseum/XD, or you'll have to complete the Hoenn Dex in Emerald three times. But if you want to do it, you'll be doing a lot of trading. Good luck.