Z4 M Coupé most definitely. The only reason I didn't buy it when it came out was the fact that a new M3 was on the way...
It's definitely the best handling and most fun BMW that's on sale right now. My only beef with it is that they could have modernized the engine for it... But that may just indicate that it will indeed get the new M3's V8 when the time is right (in 1-2 years from now perhaps), and it was financially more sensible for BMW to just keep producing the S54B32 engines for it for now, since the old E46 M3 is not for sale anymore and they don't use them up.