As a compulsive list-maker, this is the type of topic that could have me wasting half a day compiling an ever-increasing list of favourites and having to force myself to stop at about 500. With film scores, though, I tend to think of them almost exclusively as components of their respective films rather than separate entities (I own very few OSTs), and so I find myself essentially just going through my favourite films and picking out the ones that have notable scores.
So: Taxi Driver, Citizen Kane, There Will Be Blood, The Red Shoes…