Definitely an evolution involving any Eeveelution because a normal-type like Eevee may evolve into 7 different types of pokemon, which is awesome :L
i like when missingno evolves into jigglypuff
but seriously it would have to be ...hmmm... its too hard to say...well i like driflim, it's be one of my favorites...
Chimchar to Infernape, and on that note, Tepig to Enbuoh.
A little goofy Pokemon (a fire-butt monkey, and a silly pig), evolving into two badass Pokemon based off of Journey to the West. It is nothing short of awesome. I love the story so far, being only on chapter 47 out of 100. But now that I know of that story, I can enjoy the relationship and similarities between the two even better.
I don't care that Enbuoh is fire/fighting, too. As I have said, only truly worthy and awesome Pokemon are deserving of the title. Hence, why Blaziken, Infernape, and Enbuoh are that type. Haters gonna hate~
Torchic to Blaziken has to be one of my favs because, come on, an annoying little chick evolving into something as badass as Blaziken is. I love especially its voice in the anime. It has that "I can kick your ass"-tone in it.
Abra to Alakazam would be another. I mean, Abra is great and Kadabra even greater, but eventually they'll become this combination of great wisdom and power... With mustache.
Then there's Magikarp to Gyarados - that should be clear without explaining.
Vibrava LOSES Attack power and gains a x4 weakness to ice...