What is the best way to protect frame from dents during crashes?
Your bicycle is probably as protected as its going to get when it comes out of the factory, so I'd suggest doing nothing (well, maybe crash less). 


Protective Film -- Cosmetic protection (possibly structural in some cases for things like cable rub as pointed out by Nathan in the comments). If you're worried about keeping your bike shiny and scratch free I suppose. Be careful with Carbon Fiber depending on adhesive. Won't protect from dents. 
Derailleur guards are a terrible idea -- the force transmitted in a crash will be applied in a way not in line with design and damage the frame, whereas if the derailleur is installed as normal, the hanger will bend (or break) and is usually easily replaceable or reparable (align with something like the Park Tool DAG). If the derailleur is damaged, it's a heck of a lot cheaper than repairing the frame damage caused by a derailleur guard. You could spec an internal gear hub or single speed instead, if you want to avoid derailleur damage depending on the frame.
Bumpers -- See point 1. I see these as likely less useful than the film. 
Crash Guard -- See point 2. Plus, there aren't as many nice mounting points as on a motorcycle since you have to pedal on a bicycle for it not getting in the way, whereas you only shift+brake with foot movement on a motorcycle.