What would be a good Opeth album to get after listening to The Human Equation?

I would say Still Life as well.. it certainly is the "lightest" sounding Opeth album, aside from Damnation. Its also one heck of an album, with plenty of memorable clean singing, and really dynamic songwriting. The lyrical aspect is also there, since its a concept album with a single story. Good luck!
 
I agree with everyone saying Still Life, but you should know that all the other Opeth albums primarily contain growling (especially Orchid as someone said), but nearly all songs show off at least some clean singing.
 
The Human Equation is the best disc of 2004. Period.

Get Still Life. If not, get Blackwater Park. You're going to dig them. Akerfeldt's singing on these albums sound nothing like his work with Ayreon though. Those screams on Day 12 on THE are out of this world.