Best M3: E46 M3... as much as I like the E30, E46 is a beast.
Best M5: E34 -- It just has something sexy about it... plus it's rare. E28 has the same coolness factor, but E34 is a little better performer, which is why it takes the first place. E60 and E39 are amazing machines and will outperform those 2 other M5s, but it's not all about speed...
Best series: 3... affordable, great handling, etc.
850csi ... cool because I'd kill for one.
E46 m3 csl -- again, a beast.