For me it has to be Umbreon.
While I was first playing Pokemon silver Umbreon/Espeon had not been in the Anime so I though Eevee could only evolve using stones. I was amazed when it evolved into an Umbreon. I ran and told my brother, he didn't believe me so he trained up Eevee and it evolved into Espeon, which shook us both, a Pokemon evolving into a different one with out stones? Mind = Blown