Consider by many the "black sheep" of the series, Golden Sun Dark Dawn is a game that has a lot of potential wasted by not having its story completed. It feels rushed, like the developers had to finish it quickly and not fully realize its potential.
Being the third game in the series, goes to a different direction with the art style. It is not as pretty as the GBA games, I think, but it is also in 3D, which may be the reason it does not appeal as much to me. Still, of the three games, this is the one I've played the most.
It still has the amazing soundtrack, class system and combat, but the combat gets a small addition that, while I enjoy a bit, has a double edge effect: now weapons can have multiple skills they can unleash in battle. I do like that normal weapons have more of an utility, having their own unleashes, but it does undermine the "specialness" of special weapons, making them not as good, since there are some very weak unleashes, compared to the good ones.