My best experience with a 15" street machine tire has also been the Firestone Firehawk line (SS20 superceded by the Indy 500). Have had them on my 83 'doba and my 300, VERY pleased. I just got a set of 15x8 Super Coupe Rims for my challenger, and yes, they'll be wearing 255/60 Indy 500s when its time for street duty.
I also like Coopers, sorry to see they disco'd the GT Qualifier (had a set on my first car...). In low profiles, I've had great results with toyo, yokohama, pirrelli, and bridgestone, but nothing in a softer sidewall. My dad used to swear by Kelly's and Yoko's on his trucks, and when I worked at a tire store, we sold TONS of Yokos (Toyos were the brand we were told to push, but everybody wanted Yoko's)
just my 2 cents.