Question:
What is the best way to hold the pick when performing fast alternate picking?
Answer:
Well, I have seen it done differently so many times, it becomes useless to worry about a ‘best’ way. Shawn Lane would tilt the pick up, Paul Gilbert tilts the pick down, and Marty Friedman is all messed up. 
What you will want to do first is find a comfortable way to hold the pick. Start from there. Also, experiment with how much of the tip is exposed while alternate picking. Some players are good with just the slightest bit of tip exposed, while others prefer more of the ‘meat’ of the pick to be exposed. Your best choice will be based on the music you play, the techniques you use, and the overall comfort of the thing.
Beyond all of that, I have one last piece of advice: stop worrying about how to hold the pick, and just focus on playing your damn guitar!
Get practicing!
T.M.