Double's henshin is probably my favorite, I get a big smile on my face every time I see it. Plus the jazz music and the materialization of the suit are really cool. A close second would be Blade or Decade, I love Blade's build up and how the card can be used for defense, while Decade's has some great music.

Thinking about it some, I love Kiva's transformation. The stained glass effect that appears on his face when he's bitten by Kivat, his body morphing into Kiva kind of like he's a Fangire, and finally the shattering into color. Really cool ^^