hmmm... different films for different moods obviously, but i'd probably go for some like it hot for comedy - perfection as far as the humour goes, plus marilyns greatest role. the maltese falcon - not a single scene or line wasted, career best performances from the entire cast - it's about as close a film adaptation can get to realising hammet's book on celuloid. shame no-one could ever do it with chandler.
but my all time favourite film has to be pt anderson's magnolia... most films with such an epic scope (& length) struggle to hold my attention, but magnolia held me rivetted for the entire 3 hours. tom cruise is amazing, the only film i've ever seen him in where he's even bearable. william h macy, philip seymour hoffman & jullianne moore give their usual virtuoso performances, and the many interlocking, overlapping plotlines fascinate instead of confuse. and the bit in the middle where the entire cast sing along to an aimee mann song makes the hairs stand up on the back of my neck.
honourable mentions go to most of the coen brothers movies, esp. oh brother where art thou and the big liebowski
thumbs up for performance here too.