Definitely generation V. Great environment and region, tons of new Pokémon, great new features added, great storyline, a villain team with goals that actually made sense and didn't make the main bad guy look stupid, great legendary Pokémon and lore surrounding them, seemed to be marketed to a more mature audience, great characters, having two rivals that were your best friends that made it feel like you weren't on your journey alone, and many more. I loved generation V to death. I also loved generation VII, but I technically don't consider that "old" as if yet, but in a few more years, I definitely will and it would be on my list.