Sonata very easily.
I always forget about this album despite being very well versed in it since DT/SX were collectively my top 2 bands for like 3 years. I'd say this album is for SX, what Deliverance is for Opeth.
For SX, Divine Wings is widely considered their best effort and Twilight, while very good, is generally considered to be on a lower level. It was then followed by V which is basically considered to be a close runner-up to Divine Wings. Then came The Odyssey, which also made a big splash, brought in many new fans, and marked a change in style.
Similarly, Opeth's Blackwater Park is their consensus favorite. Deliverance didn't garner the acclaim that BWP did but it wasn't bad by any means either. Next came Ghost Reveries (technically, Damnation came next but it was a companion to Deliverance) which brought in a shitload of new fans and is basically the 1A to BWP. Finally, we have Watershed. It was a dramatic stylistic shift which cemented keyboard as a prominent fixture of their sound and marked their highest charting at that time. For bonus points, we could even draw a parallel between Paradise Lost and Heritage in that they both continued the bands' ascent to higher levels of commercial success despite having a very lukewarm reception from longtime fans.