Storm's Ice Storms (Black/Silver and Purple/Silver) are pretty high up on the hardness scale.
Brunswick's Viz-a-Balls are also very hard. My friend's has lasted a long, long time. You can't sand these because it removes the label and you will have a totally white ball.
Personally, I have found that using a polyester ball, one pound lighter than your strike ball, allows you to throw it much harder and get less hook from it. I am currently using a T-Zone, because that's what my driller had in stock.
I find most plastic balls tend to crack around the finger holes just from use. You can have that area plugged and redrilled for more longevity. Don't be afraid to sand and repolish any plastic EXCEPT for Viz-a-balls.
If you throw it slow enough and with enough revs, even plastic balls will hook, especially as they wear and a track begins to show up. It's up to you to throw it straighter, if you want no hook, literally, or to shine it up a lot. As for polish, I'd suggest sanding it to at least P4000 grit and putting as much polish on it as possible.
