Which game should I get first!?

[GooseGaws]

I would go with Chrono Trigger. I've played through it more than a dozen times and I literally can't wait to buy it again. I'm a sucker for that game, though.

Wait until you've finished ToS to pick up the sequel, as you seem to have decided.

FFIV DS is also fantastic, by far the definitive version, and easily worth purchasing, as is TWEWY. But if I were to choose one title, it would be Chrono Trigger. Get them all eventually, if you can.