I've tried various boards at various times. Most of them are junk, honestly. The ones I've found the best are
THESE. I'm not endorsing this supplier, it's just a place that sells the right brand. The green board is equivalent to a standard 12x10x1" board. My super scientific test has been to have someone mount them on a holder, hang a towel across them, and break them. And to also break a few boards at the same time. I honestly cannot tell the difference, even after breaking them thousands of times.
The old Bruce Lee line "boards don't hit back" isn't really true in my experience. Back in the seventies we used to go to the lumbar yard and order a boatload of pine boards cut to breaking size. Then we would go play in the dojo after class on Saturdays having all kinds of fun.
So we're breaking with all kinds of kicks and punches, acting like the kids we were. Me and another guy are holding three boards for a jump back kick. The kick was a beauty - but the boards didn't break. We tried time and again. Finally did them one at a time and found out the problem. One particular board. Didn't have an unusual amount of knots, it was just a board. Or so we thought.
We had some big strong guys who could really smack. Nobody could break it. Not with it on cinderblocks with a downward punch, elbow of foot stomp. It just would not break. We put shoes on, boots actually, it was winter, and went nuts on it. Nothing.
One of the guys went to the trunk of his car and got a small sledgehammer. We laughed. He smacked it and it cracked - but did not snap in half. We all studied that board and bent the crack back and forth until it gave. It finally came apart, sort of. It was broken, but not separated into two pieces. Upon further examination, the grain of that piece of wood was much closer together and mis-colored where it snapped. It was as if you took the knots from wood and lengthen them into a grain. It was really weird.
And as for the breakaway boards. We bought a few of them for the dojo. But there was one, of the exact same type as the others, that just didn't want to break. People were hurting themselves. So we threw it in a closet with a "do not use" sign taped to it.
Years later, after I had become a DT instructor and established myself, I brought it into my department and gave it to my boss, who was a wild man and former boxer. He hurt his hand SO bad on that puppy, but he kept it and would bring it out every year or two so others could hurt their hands as well. (funny, but irresponsible and stupid)
What I learned from all this was to really take close looks at what people are trying to break. And that learning paid off. I was overseas fighting, and Billy Blanks was going to do a big breaking demo at an evening show. He asked the folks to get him a dozen patio bricks. The day of the show I examined one. The weight was off. I smacked it against the corner of a cement wall. It cracked, but didn't break. So I smacked it harder. It was reinforced with three pieces of rebar. So much for that. I like to think it was an accident, but I don't think it was, I believe it was intentional. Asshats.