Well, GR is my favorite albulm from Opeth. I like all the songs on it equally, and on each other Opeth albulm, there's always ONE song I find kinda boring or just not fitting. (but not bad). Also, in my opinion, it has the best concept, best artwork, is the "evilest" sounding one, has the best song progressions, has the best songs in general, and none of the songs seem like wasted times, they're all pretty long and filled to the rim with fast complex parts. (cant say the same for Watershed). It was also the first Opeth albulm I had ever heard. Also, It has the best keyboard compositions. (Watershed lacked them alot).
Production wise though, Blackwater Park was my favorite. I know Steven Wilsons writing and producing, and he always adds tons of little guitar effects and vocal effects and just a bunch of tiny little things that you dont notice at first, but makes it sound amazing. I honestly think he would have made Watershed alot better, it seemed like the current producer gave it his all for GR, and didnt really add anything new for WS.
Still Life is probably my 3rd favorite, because I was addicted to it for the longest period. Besides the songs The Drapery Falls and Ghost of Perdition, Still Life has the most epic tracks I think, the best solos, and its cool because it has a sound that sounds more fantasy/tragedy than the newer ones.