In my opinion, Opeth has gone through three major periods:
1. Orchid/Morningrise
2. My Arms Your Hearse
3. Still Life through present day
As brilliant as the first two albums are, they really changed their sound with MAYH - it's darker, denser, more mysterious and more brutal. And after boldly and successfully withstanding criticisms for an all-acoustic track in To Bid You Farewell, MAYH is even more confident in its dynamics. It's also a concept album, and a very captivating and cohesive one, at that.
Still Life is an amazing album too - they all are - but I'd say there's a bigger shift in execution and writing between MAYH and SL vs SL and its subsequent albums (with Damnation being an obvious outlier).
So that's my .02.
And if I had extra $$, I'd start a foundation for fans looking to hear the rest of Opeth's discography - 'cause you really shouldn't buy anything else until you've heard it all. (and no downloading either!!!)
-GG
np: Sigh - Hail Horror Hail