I first started with 8 (on PC), and I did briefly try 7 (on PC) but didn't give it much attention. Yes, I do regret that, and would gladly get 7 again if I could still find it (legally) any where.
Later I finally tried X and X-2.
So I've only played 7, 8, X and X-2. I haven't had the honor of playing any of the older ones yet.
I know there's one that was "updated" that comes with an "updated" Chrono Trigger. Maybe I'll hunt that one down.