I'd definitely go Ars Magica (I mean, duh, look at my sig line...), but probably for different reasons than might be imagined.
I like BRP a lot, but I have always disliked the magic system. Other than that, it is a great, useful, and flexible system. It would become my "fallback" system in this lot.
D&D is, well, D&D -- fine for what it does, but it doesn't really map any fantasy setting that I know of other than D&D. C&C is pretty much D&DLite, so more or less the same problem.
As for AM...
I am busy working on extensive notes for
Regency Magica, putting AM into the world of
Jonathan Strang & Mr Norrell, along with dashes of Walpole, Radcliffe, Byron, and Patrick O'Brian. Also, due to recently supplments with the concept of the Mythic Companion, it is possible to expand the world so it is less magocentric and quite heroic. I have a friend who is mapping out
Ars Atlantis and we have both played around with other settings. AM is better at depicting the kinds of fantasy setting I prefer -- there is magic, and it can be powerful, but it is also rare, many people do not trust it, the notion that people have not only great abilities, but also deep foibles, etc. Yeah, I like this system.
Ultimately, however, I favour it simply because I am terribly comfortable with the system after years of playing.
