a lot of obvious choices for this topic but i just recently saw Walk the Line again for the first time in years.
Joaquin Phoenix and Reese Witherspoon as Johnny and June were both perfect. that film would not be the same with anyone else in their place. especially with them doing their own singing. Both nominated for an oscar, Reese winning it, Phoenix won the golden globe. sure Phoenix doesn't "sound" exactly like Cash, i don't know who would, but he captured his essence and his look/mood etc. 10/10 casting.