I'm not a massive fan of the series, but when I think back on it I've actually played and finished most of them. I think it's down to a mixture of most of the games being really easy to find for quite cheap in any Japanese bargain bin, and them usually being a safe bet to provide a fun forty to sixty hours of gameplay between titles I care more about. I'd be hard pressed to call many of the games in the series fantastic, but a few misfires aside it's got a pretty good track record with me for providing fun (if rather forgettable) titles year after year.
Off the top of my head, I've finished Phantasia, Destiny, Phantasia PS1, Eternia, Destiny II, Symphonia PS2, Rebirth, Legendia, Abyss, Destiny PS2 and Vesperia PS3. The ones that have stuck with me most have probably been Destiny II, Destiny PS2 and Abyss. The first because it was a cute, adventurous throwback to a setting and cast of characters I was very fond of during the Playstation era, the second because it's just an all around fantastic remake, with some spectacular 2D spritework and what is by far the best gameplay in the series in my mind, and the last because in spite of my usual indifference to the stories the series spins, I actually found myself getting really caught up in the struggles of Luke and his companions, and was eager to see what would happen next to the very last moment. It was total soap opera stuff, sure, but it was compelling, in its own way. :'D
About the only one I really didn't enjoy was Legendia. Though I'd offer sparse praise to some of the character quests, and I absolutely swooned over the wonderful soundtrack, which for me was far, far beyond any of Sakuraba's work on the series.