You have asked a question that sounds reasonably simple, but is actually harder to answer than you might think.
What makes a ball great really isn't something that I think is quantifiable. If it were, making great balls would be as simple as plugging in the numbers and figuring out which way to go. Everyone knows it isn't that simple.
A ball that's great should cover a wide variety of conditions, and do so for a wide variety of bowlers. Balls that have done so in the past have usually become referred to as "great" balls.
A couple of the balls that jump to mind when talking about balls considered greats, are the original Brunswick Danger Zone, and the Ebonite Vortex2. Both of these balls were pieces that worked for a large portion of bowlers, and did so over a wide variety of lane conditions, and seemed to work just as well for crankers playing deep inside and strokers playing the twig. I can remember when it was hard to find my ball on the return because, out of the ten balls up there, eight of them were probably Danger Zones.
But, just having a ball with those qualities does not ensure that it will fall into the category of greatness, because a ball needs to be popular and recognized by the bowling public as being superior, and that can only happen through exposure, and this is something that is becoming harder and harder to do, especially with the market saturation we are seeing today.
What makes a ball great? Here are things I think are important.
P.S. For what it is worth, I really don't feel like most big manufacturers even allow balls to stay on the market long enough to become "great". Couple that with the fact that the big manufacturers expect their staff players to throw the "new" stuff on the TV show to bolster sales of the latest versions, and it begins to become obvious why nothing stays around and visible long enough to be considered "great".
An exception would be the Storm Hy-Road, and that is a ball that Storm has been trying to find a way to kill off for two years now by trying replacement after replacement, all of which have failed to perform well enough to kill it. In the old days, this ball would've been a contender for the "great" term.