I've bought many a tire (and wheels) for each of my C Classes, usually one at a time due to potholes and other aberrations on our notoriously crappy roads here in the greater Boston area. I actually prefer the ride of my coupe when I have the snow tires on, as they're mounted on 16" wheels as opposed to the 17" the summer tires are (I've considered trying to swap even my 17"s for someone's 16"s, but that's another thread for another time.
When I recently had the snow tires taken off and the summer tires put on, the car was pulling hard to the left. We had put the tires with the best tread on the rear, and this left a tire with a sidewall bubble on the left front and one with little tread on the the right front. For now, I've rotated those to the back, but I can feel that low-tread one at highway speeds, and I guess I'll take everyone's advice and replace the sidewall bubble one at the same time (oddly, it seems to ride fine, but even strangers are coming up to me in parking lots.)
I've always just bought the OEM tires, mostly 'cause I'm buying one at a time and want them to match, but I wondered if anyone had a recommendation of a 17" tire that rides any better than others (I note that Luke's sticky above is all about performance and budget, not comfort, which is what I'm asking about).
The Continental OEM tire is the ContiSportContact:

(part number 245WR7SPCBMW, which Luke tells me is BMW OEM).
On my C320 4Matic, I have this other Continental, ContiSportContact 2:

(part number 245YR7SPC2V2)
These are a bit more comfortable, but not much. I should also add that I run the tires with higher than typical air pressure, which contributes to the hardness of the ride. At 35 psi, I was losing tires to sidewall damage at the slightest bump, so I upped to 45 psi.
Since I run with winter wheels and tires in the snow, I just need a comfortable riding summer tire.......suggestions?

 

if you truely want the best ride quality get away from performance tires with high air pressure. A tire liek the Turanza Serenity would be a good choice but with all that you have said why not get a 16" wheel and tire package.

 

You say you're running 45 psi in your tires?

What's the recommended cold pressure psi? It's probably on the door sill.

I've found on my C300, that the high performance tires are very sensitive to air pressure when it comes to car ride. Front is 28psi and rears 33psi cold.