VIII is also my favourite mainly because it was the first one i played.  I also played VII but i couldnt get into it although i have heard that its storyline is far superior to all the others.  I have the anthology which is IV and V or V and VI and have played them for a couple of hours.

Yes, I do have X and it is a bit different to the others because only one person can summon aeons (GFs) and the side-game is very cool.  Its an underwater sport that is a cross between soccer and waterpolo.  The aeons are different too because instead of just being summoned and attacking, they are like a character and can cast magic, attack normally and their limit break is their big move.  I havent finished it yet because im stuck at a bit  but its worth playing.