Hey, Simon:
The ball I found that has worked best for me is the Nike Nike 20XI--X RZN (L81+). But the Nike 20XI--X (L71+) balls are just behind it. From my experience, both are excellent and play very smoothly on my iPad. While I'd be happy with either, I like the little slower meter speed on the L81+.
For the longest time, however, I played the Callaway Chrome Soft (L33+) ball. I only switched because I wanted something with a bit more distance and a little slower meter speed. Plus, I had built up a lot of credits so I wasn't as worried about the cost of balls. Nevertheless, I still think, at 250 credits/sleeve, the Callaway L33+ remains the best value ball in the pro shop.
A couple of balls that didn't work well for me were the Volvik Crystal (L51+) ball and the Nike 20XI--X RZN (L61+). For whatever reason, with both balls, the meter was glitchy for me. I especially had problems with pitching with each of those balls. I tried both because I saw people who had posted those two balls are really good. But they just didn't work out for me.
A little while back, I also tried the Callaway Truvis Chrome Soft ball (L67). The ball is really smooth and nice. For me, that ball putted very nicely. But at 625 credits a sleeve and only 2.0 dots of durability, that's a fairly pricey ball. And even though it's a great ball, it's not that much better than the Nike (L81+) ball.
So when the NIke balls go away, I'm not sure what ball I'll play. Obviously, I'll see what they replace the Nike balls with. But I certainly could see myself going back to the Callaway (L33+) ball or play the Callaway Truvis (L67+). I still have a lot of credits banked that the extra cost of Truvis wouldn't be a big deal to me.