Ian McKellen
Cate Blanchett
Kate Winslet
James McAvoy
Ewan McGregor
Jennifer Garner
Lee Pace
Viggo Mortensen
Matt Damon

If I had to choose one absolute favorite, it'd be Cate Blanchett. I don't think there's a single other actor out there who can transform themselves so thoroughly and convincingly for every single role they play. She's a goddess.