I'll take a C350.
Got the opportunity to see a white Avantgarde C300 at the local MB dealership last weekend and was throughly impressed by it's presence and improvement over the W203. Good looking little car it is, although I would still contest that the car's rear-end is a little short. Additionally, the bootlid could use some sort of lip spoiler (something which is obviously standard on the C63 AMG and improves the styling balance of the rear-end). Overall, I firmly believe it'd be the car I would be the most happy with.
While I haven't any doubts of the A4's greatness (and do have an appreciation for it), I find myself less impressed with the overall package when compared with that of the C-Class. Admittedly though, I much prefer the interior of the B8 to either the BM or the MB; it's manages to be styled (and I'm guessing -based on previous Audi's - most probably assembled) in a manor which is reminscent to a larger, and ultimately more expensive automobile than what the A4 really is.
The 3er, as good as it may be, wouldn't even be a consideration for me. Or at least until they change those tail lights which I absolutely despise. I haven't any complaints about the coupe though, but as we aren't talking about coupes...
Can't go wrong with any of the three in my opinion.
Had the Lexus IS been in this poll, it would have been decisively harder for myself to answer.