FFX, without a doubt.
X-2 was okay in some aspects. I actually thought the dress-sphere thing was pretty cool, and I liked some of the new characters (Gippallllll X3). But it was too short, and it was more mission/fanservice-based than storyline-based.
They'd have been better off making a PREQUEL to FFX, rather than a sequel, to explain the Machina War between ancient Zanarkand and Bevelle in more detail...plus, I liked the side-story with Shuyin and Lenne.