Ars Magica is, in my own Top Ten, the "Best RPG Ever". Seconds are Rêve de Dragon and D&D ex-aequo.
The combat system isn't very brilliant, as D&D's one is; but the magic... Ah, Ars Magica allows you to play mages exactly like mages should be played. And the faeries... Faeries just like faeries should be. Elves and gnomes and goblins are not plain humanoid races, but strange and impredictables creatures you never know if you'll be happy or not to have encountered them.
My only reproach: the Mythic Europe setting is hard to DM, because I, as a learned person; and my historian brother as an even more learned one is this matter, don't really like to make dramatic changes to history. It's why it's easier for our group to play in totally fictional worlds. Worlds that, like Ars Magica or White Wolf's World of Darkness, are parrallel to the real world are hard to DM for this reason.
That's why in my Ars Magica campaign, I have used a world that's a bit copied on real world, but with dramatic changes from start. This way, I'll feel less hesitation to have a revolution in 1300-and-so, massive supernatural events, a more challenging church, more hermetic influence on the world, etc.
Ars Magica is really an excellent game. The "covenant" concept is something that lend itself naturally to campaign playings, and we found very refreshing the fact of alterning play between magus, grog and companion -- sometimes in the middle of a session.
Also, we didn't found the stress roll system so harsh -- in fact, it was rather advantageous to the players. Maybe that's because our d10s were expecting we were playing a White Wolf game, but '1' are a-plenty and '10' are oh-so rare... That's maybe just us, though. However, still, the '10' needs to be confirmed by rolling another, so disasters are not that frequent.
Edit: missing portion of a sentence...