Arcana Unearthed is pretty cool -- each spell has three versions (regular, heightened, and diminished). You can use the regular spell slot to cast the regular version, or a lower level slot to cast the diminished version, or a higher level slot to cast the heightened version.
Not lots of whistles and bells, but simple to use and easily extensible. It has a tactical resource management element, but lacks the annoying accounting and the pretense of lots of flashy substance that, in play, end up being more of a burden than interesting, as is often the case in many alledgedly superior magic systems.
I'd go with the core d20 system too. I know lots of people have things they might like to see done differently, so maybe I am just easily pleased, but it works for me.
The two I prefer at the moment are the Arcana Unearthed system - templates, Hightend/diminished and so forth - and Encyclopedia Arcane: Chaos Magic - similar to Mage: The Ascencion, my all-time fave when it comes to Magic