I've only played Tales of Eternia, but I've already falled in love with the series...the battle system is a major reason why.
I own Tales of Symphonia, Tales of the Abyss, Tales of Legendia, Tales of Phantasia, and Tales of the World: Radiant Mythology (JAP). My favourites are ToS and ToA plotwise, but I kind of like Legendia for it's characters.
I'm hoping to pick up the PSP remake of Eternia while I'm in Europe this summer.
Played maybe...a third of Phantasia? And I've played some of Legendia but I couldn't get into it. I absolutely hated the character designs and the gameplay just...wasn't very fun. :\
I'm currently on hold with Tales of the Abyss...I won't finish it until I've gotten some other games out of the way, but I'm loving it and its characters.
Top with me has to be Tales of Symphonia though--that's the first Tales of game I ever played and just...wow. I was obsessed with it for over a year and it's still one of my favourite games. I'm gonna be replaying the Japanese PS2 version of it this summer and I'm looking forward to that more than playing some new games I haven't touched yet. XD;