Okay, we seen Freddy Vs. Jason that Jason in the end has Freddy's head, but Freddy winked his eye to the audience which proves that fight is not over. Now, imagine if Jason goes against Ash. The teenagers found the book of the dead in crystal lake camp, and they were reading it which wakes up Jason Voorhees. Now, he is back to kill again, and he won't stop killing because he is a killing machine. But there is only one man who dares to stop Jason, Ahs J Williams! He will fight against Jason once and for all, but who will survive? Well, only one will survive. Jason is up for a kill, and there is only one man who can stop him once and for all. Let's hope Ash can put an end to Jason because our hero is gonna need good luck. This would be a battle for good vs. evil. Tell me who would win in this fight.

They covered all that in the Jason Vs. Freddy Vs. Ash comic series.

I'd read that very, very early pitches for Freddy Vs. Jason were actually Freddy Vs. Jason Vs. Ash, but Raimi would only let them use Ash if he won at the end. Nobody wanted to make one better than the other, so Ash was dropped out. Which is just as well, in my opinion, since Jason and Freddy are from very different movies than Ash.

Anyway, of course Ash would win. He always wins; that's part of the character. He just doesn't win very well, so his victories always lead to newer, bigger problems.

Ash Vs. Predator would've been funnier.