What episode does ash meet Zekrom and reshriam?

Ash doesn't meet Zekrom and Reshiram in the anime. They are legendary Pokémon from the Unova region and are featured heavily in the Black and White games, but their appearance is primarily limited to the games.

Ash does meet other legendary Pokémon in the anime, like:

* Articuno, Zapdos, and Moltres (in various episodes)

* Lugia (in the movie "The Power of One")

* Mewtwo (in the movie "Mewtwo Strikes Back" and several other appearances)

* Rayquaza (in the movie "Destiny Deoxys")

* Dialga and Palkia (in the movie "The Rise of Darkrai")

* Giratina (in the movie "Giratina & the Sky Warrior")

So while Ash might not have met Zekrom and Reshiram in the anime, he certainly has encounters with other impressive legendary Pokémon!