1, 2, and 4 were the best ones.
3 and 5 were alright
Stay away from 6 and 7
I'm told that X8 is actually pretty decent
I want to say that the X1 had the best level design since beating a level would affect the environment in other levels (example, beating the Storm Eagle stage will cause a power outage in Spark Mandrill's stage, and so on).
X2 was underrated. It actually had some creative levels and a great soundtrack, the first game just overshadowed it.
Those anime cutscenes in X4 were comedy in retrospect