I've played both the Aerith and Tifa demos fro FFVII, though I found the Aerith demo more interesting because it had a few more changes from the final product, whereas Tifa demo was released a few months before the game and is pretty close to the original. Sadly, I own the Tifa demo but the Aerith one is really hard to come by.

I own the PS1 demo of VIII from Brave Fencer Musashi, which is a pretty snazzy demo that takes some cues from the Aerith demo in that the main heroine is available in a part of the game she isn't suppose to be and you have access to Leviathan. I still play it from time to time because I still feel that Raid on Dollet is a superior track to The Landing.

I've played the FFX demo, which is not nearly as interesting as the above ones since it's more or less just the whole opening segment in Zanarkand and the sequence when you try to leave Besaid.

I had the X-2 demo as well, which again isn't too far from the original game anyway outside of really limited customization.

I totally missed the FFXII demo. My copy of DQVIII was used and the disc was conveniently missing, so I can't comment on it.

The XIII demo was Japan only, and I wasn't going to waste money on getting the Advent Children Complete it was bundled with anyway.

I did play the XIII-2 demo to see how much it differed from the original, but realized I had no desire to continue playing the series. As you can guess it, I skipped the Lightning Returns demo as well.

Played both of the FFXV demos, honestly liked Duscae far better than the Platinum demo in terms of how the battle system worked. Platinum demo did have a lot of interesting elements to it, but I never did see Carbuncle in the main game.