To be honest, it is a tough call for me. Kaiba pissed me off with how he wouldn't believe in everything that's going on around him. Be it Japanese or English, it didn't change too much. Kaiba in the dub was just even more delusional.
It took the true shadow games in the ancient times and facing off against Zorc to realize, "...oh **** this is real."
Zorc: WHERE'S YOUR HOLOGRAMS NOW *****!?!
I love the role that Gary has taken on in the anime, meanwhile. It's rarer to see him, but when we do see him, he's nothing short of a phenominal trainer. He is up there in skill with the likes of Ash for how far he's come in Sinnoh. And he's more focused on RESEARCH for all of that. Ash conquers the Battle Frontier, legendary Pokémon are defeated by his teams, and he is offered a role as a Frontier Brain for all of his skills.
And what happens? Gary Oak isn't even an active trainer who competes for the leagues and he STILL can pull out an impressive win over his old friend. He's really matured, maintaining that confidence streak while losing the naive cocky attitude that lead to his downfall. After losing to Ash in Johto, he just became that much better of a person as he was humbled by the one person he felt he was better than.
Gary ftfw.