For me it's Fargo. I remember hating it when I say it 20 years ago, but after about a 10 year break I gave it another shot and can't imagine how I ever hated it in the first place. Now one of my favorite movies of all time.
For a little more context, I think when I was a younger person I found myself trying desperately to route for the characters in the film and found myself increasingly frustrated that they all seemed to make worse and worse choices until they descended into little hell's of their own making. I suppose after a decade of exposure to the real world and a fresh set of eyes, I was able to see the movie for what it is. A rather humorous farce that tells the story of terrible people reaping terrible rewards for their terrible motivations. And a damned well told story.
I don't think I've ever had the experience of going from hate to favourite.
But I disliked Citizen Kane on first viewing (I was too young) and now I like it a lot and can see its merits.