I think any coaster with music is my favorite.
I love Space
I love Screamin' (though the music never works for me).
I love Big Thunder (the best time is right after Fantasmic when all the fireworks smoke is on the track. It's spooky and awesome)
I have to say California Screamin'. Although it's popular to be negative towards anything DCA related on these boards (maybe with good reason), this is a world class roller coaster. It has truly everything that good roller coasters have - an upside down loop, fast accelleration, drops, tight turns... really a great coaster.
Space mountain since the re-do is a close second. One thing common to both is the use of music in the seats to enhance the ride - I love it, and Space Mountain does this really, really well.