This may sound kind of simple or obvious, but I think that a good leader is one who makes their decisions based on what they feel is best for their Clan. For example, Firestar, Bluestar, and Tallstar (in my opinion) were good leaders because everything they did was to help their Clan. Tigerstar, on the other hand, was a bad leader, because all his decisions were made (both in ShadowClan and ThunderClan as deputy) to feed his own ambition. I'm on the fence about Leopardstar, though; she joined Tigerstar for the good of her Clan and to help them succeed, but I feel like she also took some personal gain into consideration.
I feel like Yellowfang would have made a good leader if given the chance. She was firm and bitter, but she loved and protected her Clan(s) with everything she had.