All my perches are tree limbs. They prefer the bigger ones, around six inch diameter, the smaller ones are good for climbing. I don't bother removing the bark since it adds grip, they seem comfortable.
The roosts in my coop are tree branches that have the bark shaved off (they're easier to clean without the bark). Chickens really like the natural bumps, curves and twists on tree branches because they can get higher or lower depending on how varied the curves on a branch are. It makes them feel like true Jungle Fowl!