Answer:
lo
Que es "lo" mas important de tu carrera como actor?
Step-by-step explanation:
What is most important about you career as an actor?
"Lo que" would be used if you were asking "Que es lo que importA mas?" which more specifically asks, what is is that matters most. Used when you're giving options.
"Lo que mejor" is redundant; you already have "mas" in the sentence. You would be asking, "what is the best most important?"