As others have said, "stronger" is really tough to define. Are you looking for the ball that will dig into the oil the hardest and avoid wiggle, or are you looking for something that covers boards and offers a lot of movement?
Of the three balls mentioned, I've only thrown the Grudge. The Grudge is middle of the road for me in that it digs in nicely to the oil, but that doesn't sacrifice all of the backend. It's not flippy by any stretch, but it doesn't lope off the spot either.
I've seen quite a few Locks in league and tournament. To me, that ball looks super early at box finish, which is great if you want a ball that digs in hard in the oil. As for backend, most of the Locks that I've seen don't really boom all that much down the lane.
Lastly, I've seen 1-2 Alpha Cruxs thrown. To me, the Alpha appeared to be a smidge cleaner up front and a bit stronger down the lane compared to the Lock. Now, that goes against what spmcgivern has reported. I'm not sure how much he has seen these two balls compared to what I've seen, so you'll have to decide what you want to go with there.
In the end, all three are strong balls. I can tell you that I REALLY like my Grudge when there is enough oil to use it.
