There's nothing to ruin the original FFVII game. Anyone who complains and states so, obviously need to get out of the house once in awhile.

I personally loved Crisis Core, simply because Zack's character develops so much and it's quite easy to to get attached to him. I think it is a very good lead up to the original game, and I'm not sure if there'd be any better way to do it.

Advent Children isn't as bad as a lot of people say, there is quite a bit of development on Cloud's side. Granted it'd be very nice to see what they took out of the DVD release and put back into the Blu-Ray release.