I agree that Ars Magica is fantastic, but I've never actually played the game so I can't really speak from experience. It does look good on paper and I've used aspects of it in other games.
My personal favorite is the ritual magic system presented in GURPS Voodoo/GURPS Spirits. It isn't complicated, its very flexible, and it works well in practice. Better yet, it perfectly fits the kind of subtle magic/creepy occult games I like to run. I like the hermetic bits from GURPS Cabal, but since I'm not a fan of the standard Gurps magic system, I've had to try to adapt them to other systems.
I confess that I have never been a fan of Vancian D20 magic, it it is probably the #1 thing that keeps me from running D20 games. Grim Tales probably comes the closest to being acceptable for me.
Lately, I've been enamored of the idea of using Buffy-like magic in games. Not necessarily magic from the BtVS RPG (though that system is good enough for what it needs to do). Specially, I like the idea of systems where characters can cast spells directly from reference material without having to spend points on learning the spell (unless they want to get good at it or cast it from memory).