GR, for creativity, variety, and sound quality. GOP for example has about 16 different 'beats' in it, not counting all the different fills. Baying might have more. Most songs in the non-Opeth world have 3-4.
Another great thing is that these beats aren't technically difficult---they're easy enough to figure out and play. It's the creativity that stands out, which I'm sure is also facilitated by Mike's writing.
My favorite drum parts include GOP, 4:10-4:55, and Baying 3:05-3:15 and the snare work in the Martin-beat at 4:00+.