Well it's going to be hard to judge their quality if you've never played them but...
Tales games are similar to Final Fantasy in that none of them are directly related aside from obvious sequels (EDIT: actually I forgot; Symphonia was a stealth prequel to Phantasia. Only exception AFAIK). It's just cameos and recurring references and such.
After Eternia (I think), they were developed by two major dev teams - Team Symphonia and Team Destiny. Symphonia, Abyss, and Vesperia were all done by Team Symphonia. Graces F is done by Team Destiny and is the first game from that team released outside of Japan (I think their others were Destiny R, Destiny 2, Rebirth, and Hearts). It's one of the "main" teams so it's supposed to be good.
Games like Legendia and Symphonia 2 were done by offshoot teams, and that's why they're so different and have....varying leaps of quality compared to the main series.
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You have no time to ponder such questions, as Link has just grabbed a Smash Ball.
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