When I first read this topic, I thought the topic was about, "What makes a good battle system?". It certainly sounded like a discussion topic.
In saying that, I suppose a good battle system would be one that always gets you thinking, and not simply just a grind or mash fest. I personally believe it's making the best of what you have, as turn-based, action-based, any type of battle system really can be extremely well done with the usage of creative ideas... Of course, that's the obvious, isn't it? :v