I think I actually had the most fun playing that one. I miss the cheesy announcer, and until I looked up on Youtube for nostalgia's sake, forgot all about the X Cup. Plus, while GX maintains the themes for the most part, X had much more of a shred guitar/heavy metal feel to it in general.
GX is still an awesome game, and really nails the sense of speed, but I think I really started loving the series with X.