I'm a big old softie and it doesn't take much for me to start bawling like a baby while watching a movie. However, I don't think any movie has had me crying as hard as Shadowlands, starring Anthony Hopkins and Debra Winger. It's the ultimate tear jerker, just unbearable. I get sad just thinking about a particular scene.
Other movies that made me cry, for various reasons: Sleepless in Seattle, Love Story, Fried Green Tomatoes, Boys Don't Cry, Leaving Las Vegas, Bambi (I was crying so hard that my parents had to escort me from the movie theatre. I was three ). Generally, any mushy rom com has my tear ducts in hysterics. Dramas about people fighting illnesses, too.