I voted for X2. I thought the chapter 5/6 part of X3 was amazing, but it kind of fell off for me after that. My own biased ratings for the games are as follows:
X2 - 9.5/10
X3 - 9.0/10
XX - 8.0/10
X1 - 7.5/10
I like it when my mom goes out of town because I get to sleep on her side of the bed. -William Montgomery