The World Ends With You, hands down. Alone the Gantz-like story is worth to buy it. Love that game *_*
I agree, but the OP said he was looking more of a turn-based RPG though.
Exact-Psience My 2014 Top 10 Games of the Year: Monster Hunter Freedom Unite, Hearthstone, VVVVVV, Battleheart Legacy, Wayward Souls, Bug Heroes 2, Powerpuff Girls Defenders of Townsville, Deep Loot, Monument Valley, Bioshock Honorable Mentions: Traps N Gemstones, Bardbarian, Goblin Sword
If you're looking for a turn-based RPG with the same interaction you'd get with Super Mario RPG on the SNES (ie. timed button presses to boost damage etc.), one of the closest that Square Enix currently offers for the iOS would be Final Fantasy 6 (which came from the SNES as well). While you rarely have the said button presses, you do have a huge variety of playable characters, each with their own unique abilities (some require more interaction than others), and have battles played out in semi-real time (turn based with constant time progression).
If you're looking for something with less complexity in terms of story, and to an extent the gameplay, but with more interaction in battles, there's Drakerider. It uses a turn-based system somewhat similar to FF6, but on a 1 vs Many scale (you control only one character at a time). Oh, and it does have console graphics (roughly PS2 standards). It may get repetitive towards late-game, however, despite the introduction of a few more playable characters.
If you didn't have turn-based as the requirement, you can't get much better than The World Ends With You, originally on the DS. The combat system is highly interactive (and somewhat deep), and you won't find similar offerings in other iOS games. If you've already played Wayward Souls and Battleheart Legacy, TWEWY may be pretty accessible to you.
And yes, there's Final Fantasy Tactics for a strictly turn-based, isometric, tactical RPG, a port of War of the Lions on PSP, which itself is an enhancement of the original PS1 version. Keep in mind, however, that FFTA on GBA has an added layer of complexity compared to FFT, in terms of equipment and battles, AFAIK.
By the way, for the Chaos Rings series, I came close to late-game in 1 and barely progressed in Omega, but the story didn't confuse me one bit in 2, as you're working with a new setting and new characters. It's like different iterations of FF, but with each game sharing a similar theme in this case. Having said that, if possible, I'd wait for 3, as it is turning out to be quite different from rest of the series, as far as combat goes.