The best Power Armour is the T51b Power Armour which can be found in Fort Constantine and accessed using the keys from the You gotta shoot 'em in the head quest. Taking this armour finishes the quest. It is fully repaired and comes with a helmet. Note that this armour can only be repaired by npcs. The armour has a DR of 50 and the helmet has a dr of 10.
the best power armour is the winterized t51b power armour you get after finishing the operation: anchorage DLC. it is exactly like the regular t51b but its color is changed to white and never loses "health".