what is the best broad head to use on elk and why?

It won't be about the "best broadhead". It will be about the one you prefer. I like Muzzy 125 grian. Tried and true and work great for me. All of my arrows, practice tips and broadheads are exactly the same. I like to keep things simple. Mechanicals are not for me although a lot of guys use them with great success.

Any sharp COC broadhead will do just fine. If it hits where you aim, you're good to go.

I'm trying the slick trick 125 magnums this year. I've heard good things about them, so we will see. I agree with others that you should steer clear of mechanials i.e. rage etc..

A solid and sharp fixed blade broadhead. I prefer Slick Tricks.

NAP Razorbak

Rage 2-blade

Because!

Slick Tricks...Put in the Right spot and its Brown Down

Personally I'm a fixed coc blade myself, but after seeing the hole and the amount of blood loss my son-in-law got on his elk he killed in CO last year, I am rethinking. You could place your whole hand flat in the entry hole. The problem with his hit is that it was a little high and he didn't bleed outside. It took almost 3 hours to find him with practically no blood trail. When I cut into him about 5 gal of blood came rushing out of chest cavity. He also killed 2 25lb gobblers this spring with them. Tore them up! I hit 2 solid with slick tricks and didn't recover either one. Very upset.

Agree 100% with the COC broadhead choice.

ahem...NAP Razorbak...

2 blade cut on contact with 2 bleeder blades, with rotating technology.