I think it is Cameron's best film, though I think it a good movie rather then great. I can think of a couple of other movies that do the whole 'Doomed Romance Against a Background Of A Historiacl Event" better..Dr Zhivago comes to mind.
But yes, it's box office run is incredible. It had one well is several releases. including one in 2012 for the 100th Annivbersary of the sinking.
Interesting question is would the film do as well if it got it's frist release today, given how much Historical Epics have fallen out of favorm and a great many people getting bored if there si not some big epic action scene every 20 minutes.
Most amazing theing about the CGI in Titanic is not the sinking, but it incredibly accurate recreation of what the Titanic looked like. Cameron seems to have fanatic about the whole production design, down to the costumes and props ,being as accurate as possible, and spending quite a bit on research to make so it was.