I've always felt Titanic was just another mediocre love story with some outstanding special effects, but I'll admit I'm not a big fan romance on the big screen. As others have said, a lot of factors need to be looked at - inflation, number of theaters the movie is shown in and how long it stays in theaters, etc.
I've also always felt that a very big reason that movie made so much money was the popularity of Leonardo DiCaprio with women. When that movie was still in theaters, I remember reading many stories that mentioned girls going to see Titanic over and over again just to see DiCaprio. Repeat business had a lot to do with the movie's success.
There's absolutely nothing wrong with that. It's just another thing to take into consideration. Some people automatically equate "financially successful" with "great movie". I'm not knocking the movie or it's fans. I love the way they showed the sinking. I just wasn't impressed with the rest of it.
As far as surpassing it goes, it'll happen before too long, I think. It just takes a special mix of circumstances to click at once. Something to appeal to more than one demographic at once, like Titanic had romance and action and nailed both of them.