The two are both pretty close for me, but I def lean more towards Congratulations. Siberian Breaks is a fucking masterpiece :) People associate them just with their singles, but in truth.. Oracular is for the most part a psychedelic album. Songs like 4th dimensional and of moons could have easily been on Congratulations.
Also, when will people stop saying they abandoned their "mainstream" sound?? Time To Pretend and Kids were both written years before their debut. They had already moved away from that sound (as is evident from the rest of the album).. but Columbia wanted the songs on it. The progression between each album really wasn't a surprise whatsoever.