Sorry, it's just a personal opinion, but Titanic's for teenage girls, as a guy I like Avatar 10000000000000 times more.
The best movie ever? La vita è bella or Life Is Beautiful as it's called in English is my favorite movie. I think it's the most heart rending beautiful film I've ever witnessed in my life. Others may disagree though.
Raiders of the lost arc has changed my life forever that means it made me addicted to movies since I saw it 15yrs back. I also watched many movies and I choose Titanic best of all time. I have not watched Avatar. Hope will watch it soon.
Avatar, Twilight and some of the old movies like Green mile, Serendipity Fantastic movies, i have a lot more in mind but it's hard to choose, really tough!