What is the best pool cue shaft?

Predator makes a fine cue, but they're not for everyone. Make sure you try one out, or any aftermarket shaft, before buying.

If you're just looking at it from a deflection standpoint, check this website:

http://www.platinumbilliards.com/rating_deflect.php

As you can see, the I3 shaft is a significant upgrade, but still not in the same ballpark as Predator. I have heard people say they prefer the feel of the McDermott I-shafts though, so that doesn't mean it won't be the best shaft for you.

Good luck.
References :

I prefer Predator shafts over McDermott. They are more quality and have a better feel.
References :

I use a Mcdermott and I like the shaft on it really well. I have never shot with a Predator so I can't say on that
References :

I also have a Mcdermott with the factory shaft, but I bought an I-2 at the same time. I prefer the I-2. It changes the weight slightly, and I have had to replace the Moori on it, but I like the smaller tip and the shorter ferrule. It's really a personal preference. I've never used a Predator shaft, so I can't compare. One other item of interest: The I-2 I bought doesn't match up quite flush at the joint with the butt, but the playability isn't affected at all. Good luck!!
References :

the shaft depends on what you like… but i would recomend a predator shaft. but dotn get the z shaft it is over rated
References :