E63AMG W211 is your best bet. After much shopping in the 30-45k range, I've seen 'em all.
Forget the M5, peaky gas guzzling engine, superb sound yet styling isn't aging well, and reliabilty is a crap shot, VANOS will eventually fail, SMG pump will eventually fail, that's 15,000$ in repairs.
The C63AMG is actually pretty goddamn reliable, but the brakes on these things cost an arm and a leg, there are also issues with the 6.2's camshafts causing idle knocking sounds and eventually wear out the valve springs and stuff.
The CTS-V is by far the most logical choice, heart says M5...
You're the only one that truly knows your budget, go ahead and keep us updated!