The S4 gets my vote, it's head and shoulders above the other vehicles and quite frankly shouldn't even be compared to them. It's is more of a Luxury/Sports hybrid as opposed to the others, which are cheap but have good HP. An S4 should be more closely comparred to an M3.
If you really want a sex machine for a car, look at Audi's RS6 and RS4 (I think both product lines were finished last year so you'll have to find one used). My parents used to own an RS6 and love it.
IMO the new M35 and M45 are great cars from Infiniti. The G35 sports package coupe is nice as well.