We did 2" hemlock. If budget were no object I would have used maple or oak, but the barn cost a fortune as it was. I also did the T&G ash in the stall fronts and partitions and it looks lovely. Be sure to order some extra! The sales rep at System Fence told me they will not be carrying it after this year because all the ash trees are being wiped out by the emerald ash borer. Anyway, here are some pictures of my barn interior. It was just completed in December.
Former barn had stalls lined with horizontal pine 2x6's. One horse once put a dent in a board but that was all the damage they sustained.
For the new barn, we went with puck board. It is a white practically indestructable solid surface. Easy to clean which is a good thing as 2 of my horses are back wall poopers. Brown poop on white wall definitely stands out lol.