All of them
The Pikachu short for Genesect and the Legend Awakened featured every Eevee evolution, including the newly revealed Sylveon.
But, others include:
In the anime, the Eeveelutions have been featured together in several episodes, most specifically in The Battling Eevee Brothers under the possession of the Eevee brothers, which focused on the three Generation I Eeveelutions, as well as Eevee itself, and Trouble's Brewing under the possession of Sakura and the Kimono Girls, which focused on the five Eeveelutions there were as of Generation II, although Sakura's Espeon was still an Eevee during this episode. The seven Eeveelutions were first featured together with an Eevee as a small cameo at the beginning of Giratina and the Sky Warrior.
However there do not include Sylveon.