Titanic. I assume if I re-watched it, it wouldn't be a horrible movie, but, No. 2 in box office history? I think the difficulty with answering the O.P. is that pretty much all major movies are throw away, and the ones that have some merit to them don't really get wide audience appeal so don't make great discussion topics.
Yeah. I think blockbusters have their place, even if they aren't necessarily as decent as some of the smaller movies, arthouse movies, etc. [I should also note that, while I appreciate quality in a blockbuster in terms of characterization, drama, etc., I go there looking for some other things versus a dramatic movie per se. Blockbusters often utilize some decent action sequences, for example; a dramatic movie can be focused on character interactions and thus has to deliver on that end since it can't rely on other strengths.]
I was actually thinking of Titanic, I just hadn't gotten around to posting yet. I think Cameron has a high level of detail involved in his sets and the time sequence and constructing a real sense of "you are there" in the last few days of this ship. He also had a historical event already surrounded by mystique and romanticized tragedy that he was riding on.
However, aside from Jack revitalizing Rose / sacrificing himself for her and the sense of Rose losing herself by trying to regain her family's fortunes for everyone else's benefit (and some of the broad class conflicts, comparing the first-class and third-class sections of the ship, for example), the Rose/Jack romance was pretty thin, it's not one of DiCaprio's better performances (his lines typically fell flat / sounded garbled), Billy Zane is kind of over the top in spots, and Bill Paxton is miscast, among other issues.
The movie could have been much more solid versus coasting on the set/timeline detail, so it was disappointing compared to how much it might have been. Still, box office is more about whether you can get repeat viewings plus hit the largest demographic; it can be somewhat correlated to quality (a really crappy movie usually won't get the numbers out for a prolonged period of time) but not directly.
Cameron had similar issues with Avatar, even more directly related to the script. Again, he had some great actors who brought oomph to their roles (Zoe Saldana and Stephan Lang are the main two I'm thinking of -- I think they were really good there), but when you miscast your lead (Sam Worthington really is a lousy actor, even if he's got action chops), rip off an old storyline we've already seen, and waste some great actors (like Giovannia Ribisi) with really crappy lines, well... yes, you made a ton of money mostly through the spectacle of the set and the experience of 3D (which was novel at the time) but not a movie that will endure as technology improves and it's one really strong saving factor is thus eroded.