Evolution, 0:41 (verse) - Great riff with dynamic breaks. No matter if a riff is fast or slow, breaks make it destined to be bad ass.
Fallen, 0:33 - As I was saying about breaks making riffs badass.
Fallen, 5:16 - This riff is great. I love during the third repeat when the bass plays a few little fills. I have to hand it to Lepond; most rock bassists wouldnt even think of playing something different than the guitar during a crunchy riff.
Communion and the Oracle The main groove, in 5/4. Probably the best Symphony X groove I can think of.
Egypt, 0:52 - What a chunky, fast riff. Although its a subtle difference, I love how on the repeat of the riff Romeo double-chugs the Ab. Its barely noticeable, but it builds anticipation of the verse.