Asking about the best break tip is like asking what the best car is. It's totally subjective. Phenolics and thermoset tips will not compress as much, so there will be less energy lost in compressing the tip. However, that also causes a slight lift in the cue ball. Put a line of dimes on a line to the head ball in the rack. Hit a break with a phenolic, or super hard tip, and you may find the cue ball was airborne the entire distance to the head ball. With a leather tip, it will compress more, and the downward strike won't make the lift as pronounced. You may hit the last few dimes. You don't transfer as much energy to the rack if you're hitting the head ball above the equator, because physically, some force is displaced in a downward direction into the balls behind it. Slate isn't perfectly elastic, so you will lose transferred energy.
The reason for a super hard tip is to increase velocity off the front end. I don't necessarily think that a 30 mph break is better than a 23 mph break, unless you're as accurate at both speeds. Trust me, you can't be. Let's say I can hit the head ball square every single time at 20mph, but only hit with 60% full transfer at 30. Math time. The first break will transfer all of its energy into the head ball. The second will transfer 30x0.6=18 mph effective breaking force. I don't see any huge advantage to a phenolic tip. If you're amongst the elite, and can control it, there's definitely an advantage. If you're not at that level, the tip will not be a huge deciding factor in how well you break. Your mechanics will. David Howard broke huge with a bullwhip Meucci with a Le Pro tip and soft plastic ferrules. Shane breaks like a monster with his playing cue. Figure out what works for you, and stick with it.
I like a good pressed Le Pro as a break tip. Squish it in a vise, and it's good to go. Holds chalk better than the composite tips, too.
