tough to say because they are a very consistent band. Probably SC, BC&SL, ADTOE.
Systematic Chaos is a very good album, for the first half. Then there's Repentance (yawn), Prophets of War (not terrible, but just ok), and Ministry (yawn). Even the first half of ItPoE - Part 2 is a snoozefest. The album ends pretty well though, eventually.
Black Clouds is a mess. Songs are too long, sound is too dark, lyrics are too literal. Not a lot to like (The Count of Tuscany notwithstanding).
A Dramatic Turn of Events is another case of half good, half boring. At least they spread the love here. First track is pretty good, then snoozefest until Bridges in the Sky. Outcry and Far From Heaven are nice. Breaking All Illusions is ok at best. I like the last song a lot.
And, for the record, I think Octavarium and DT12 are two of their most consistently good albums.