Okay, I'll just completely edit this for you.
I'm gonna be blunt and Straight up, Neither of these moves have priority, Ever.
Keep in mind that Accelgor has a base speed of 145 to Zekom's 90. So even At a ridiculously high level, there is still a good chance accelgor is faster.
And for the Mewtwo Liepard situation, Are you sure he isn't using Sucker Punch? Otherwise, Your game is just glitched.