M3 - e36
I have loved the e36 m3 for years and will continue to do so for the rest of my life. Now granted the e30 m3 is the most trusted for performance means and i would still give my left big toe for one. But the e36 has something more, i think it is one of the most attractive cars bmw ahs ever made, you can mod the shit out of it, and it performs like a dream right out of the box while still being a comfortable car to drive.
M5 - e34
I love the e34. Last 5 series to hold onto the more rigid body style, and they can perform like gods. The e34 m5 is easily one of the most beautiful m5's bmw has come out with.
Series - 6 series
I love the 6 series. not the new 6 series, the old one. My dad has an 87 L6 and i will not be happy till he repairs the damn thing, i remember when that thing ran and it could haul so much ass and handle like a beast.
CSL - e46
Watch the episode of Topgear that i think Jr posted in the multimedia section of the e46 CSL. that thing is fucking incredible. It sounds amazing, is faster than a bat out of hell, and is performance at its best, it doesn't even have AC for crying out loud, and the boot floor is made out of cardboard.
The 850CSI sucks. Because I don't own one!