I've been back and forth trying different things in my pens. I still haven't found the perfect thing. Perhaps pine shavings are, if you rake them out and replace frequently, but that gets expensive. Sand is nice, and I've bought a truckload of it--8 yards gets me free delivery--but where the truck access is to dump it is far from my pens. It is heavy and hard work, esp. in the heat to wheelbarrow loads to the pens. And I can't easily move the wheelbarrow INTO the pens to dump it, so that means shoveling it into 5-gal buckets that I carry and dump

Ugh, I'm tired just talking about it. I do like using sand, as it is absorbent. I use it to fill the huge holes that the hens dig. Then they kick it out again. Some of my pens look like craters on the moon, there are so many pits. Hard to rake out with all those holes.
Other things I've tried in my pens:
Straw. Don't like. Compacts down as soon as it rains and the birds eat too much of it and it has no food value. I worry about impacted crops with them eating it. Some people say it can be a vector for lice/mites. Don't know for sure about that.
I use a chipper shredder for yard prunings and have put this material in the pens. It is a mix of coarse and fine. The birds love it and pick through it and eat material they like. It makes a nice mulch in the pens and is free. Eventually, it needs to all be raked out, because as MagicChicken said, it doesn't compost well here. Raking it all out with all the accumulated poo is a lot of work, but once mixed with the chicken manure and put in a deep hole and watered, it will compost better with the added nitrogen.
I have a lot of growing birds and they eat and poop a lot! I like to clean the pens more frequently with growing birds, so having litter in the pens is a waste. Right now the pens are just dirt since it is easier to rake them clean. It seems like less flies when there is something in the pen to absorb the moisture. I did put several wheelbarrows of chipped/shredder material in a 6'x12' pen that has growing chicks and it is nice for now.
I turned off my mister this morning because too many wet spots in the bare pens and too many flies. I hate flies

Chickens did okay today without it.