I am not going to even consider any "battle systems" outside of RPGs solely because we're on an RPG Maker forum.
For me, I make two distinct categories in RPGs: real-time battle systems and turn-based. And for real-time battles, my favourite is definitely Kingdom Hearts (sure, it's camera wasn't perfect, but it didn't mar the overall combat for me), and for turn-based it is Final Fantasy X.
Kingdom Hearts had excellent enemy and boss design, and the innate mechanics and capabilities of the player made you feel really good for playing skillfully. Player positioning was huge, and so was the idea of learning the boss's attacks and how to deal with them. You can dodge roll, you can guard, you can deflect, or simply just jump out of the way, but you have a lot of ways of dealing with enemy attacks that the Tales series or Star Ocean series do not allow for the simple fact they have either no jumping at all (Star Ocean) or extremely limited jumping mechanics (Tales).
I don't buy people's argument of "but I just mashed X attack the whole time to win" for KH. The game inherently doesn't put much emphasis on things like Artes from Tales games or Battle Skills from Star Ocean. THE CORE GAMEPLAY is using your normal attack + using your gameplay space wisely + reacting to the enemy's attacks. The game doesn't ask you to be skilled in performing combos, it asks you to be skilled in other ways.
This is the distinction between KH and Tales. Tales games have horrible boss design, but the game asks you to be skilled in combo execution. Tales games give the player a high skill-cap in execution, meaning if you dedicate yourself to experimenting with a character's skillset and try to come up with a giant combo, you can do so and feel very good about it. However, as I said, their enemy and boss design is lacking severely.
So Tales boss design. I think the inherent mechanics in every Tales battle systems simply do not allow for the same level of strategy in a Tales boss fight as in a KH boss fight. Tales boss fights generally have horrible tells for their attacks, they are HP sponges, and they have no inherent way of dealing with infinite combos and so have to integrate 'Overlimit', which in my opinion, is a poor way to dealing with their problem. Boss can't be staggered? Oh, and the boss doesn't have much wind-up before his attacks? Yeah that's a poor recipe for good enemy design. I can appreciate what Xillia 2 tried to do to counteract this problem with their Elemental Weakness system -- it was a good attempt, but it still falls short. I mean, just look at the range of bosses or enemies in Kingdom Hearts. They don't have to make enemies stagger-proof because they give enemies the tools to get themselves out of infinite combos. They teleport to safety, or they do a giant back step, etc. Combo Riku in Destiny Islands too long and he falls down, and sure, you can continue to attack him while he is in this state, but he's about to perform his strongest attack at you, and so it is not smart to simply continue mashing attack on him. Wakku doesn't stagger from combos unless you stun him with his own blitzball, and then he allows you to perform one combo on him before he'll stop staggering again, but in this case, it's all about windows of opportunity to attack.
Okay, enough rambling. My main point is that the KH battle system and the Tales battle system cater to different things, and I happen to appreciate what KH does A LOT more than what Tales does. I appreciate interesting boss fights much more than simply learning complex combos, and I have a hard time excusing Tales' bad boss fights in lieu of their deep combo-based system.
And Star Ocean? Oh boy, while they are fun games, they have very little depth. When the only way to deal with Star Ocean 3's super boss's ultimate attacks is to use a Stun Bomb on them, you know you have a horribly designed attack. Seriously, each of Star Ocean 3's super boss have an ultimate attack (there is a good wind up to let you know it's coming) that HITS THE ENTIRE BATTLE FIELD AND IT WILL ONE SHOT EVERY ONE IN YOUR PARTY. So you have NO choice but to prevent the attack completely and use a Stun Bomb. Star Ocean 3's final boss has a MUCH better designed ultimate attack -- it's an attack with clear hit zones and so you can avoid it throughout careful positioning. This is a key necessity in real-time battles.
I've played through all of them, FF2 seems to have the worst one among FF games. We all have different tastes, but what I remember about FF2, it has a lot of great ideas in the gameplay, but lack of implementation and planning. For example, idea to have a system without levels was very unique and good - you use physical attacks in the battle, strength will be increased, you use magic, magic will be increased, you lose current HP, your max HP will be increased. Generally the system was ok for physical classes. But the magic system was completely ruined. You need to use 800 times (!) a spell to reach the max level. Spells as Haste misses your characters during first 400 casts 
