Jaguar XK R Perfomance or S-Type R, both have smooth automatic gear changes, beautiful sounds and look great. 👍 Just a shame they got the usual standard pixelation.
Another very interesting car is the Pontiac GTO from 1965. Stock power is more than enough, the stock transmission is good for cruising, it has nice stock wheels, and it has an insanely soft suspension. Try it with brake balance 0/0, but be prepared, it takes longer to brake than to accelerate...
About the GT86 which seems popular in here, I agree, I use my FR-S a lot, though I never found the stock one really exciting until I got my G27. On controller it's a bit dull, maybe help it with power upgrades.
More interesting cars you could try:
Lamborghini Miura/Countach LP400
Nissan GT-R 2001 Concept, make sure it's the old standard one, as it has a 7 speed automatic with smooth changes, and an extremely cool black stripe on the roof.
Daihatsu OFC-1 Concept, has an automatic 7 speed too, pretty quick acceleration.
Aston Martin DB7/DB9/Vanquish/V8 Vantage/V12 Vantage, all these are great cars on their own, you should try them all if you haven't yet.
Ferrari 599 GTB Fiorano, has a nice sound, my favourite of the Ferrari GT5 line-up, others like the 458 are amazing for sure too.
Ford GT40 Gulf Design Race Car 1969, dat sound!
Infiniti FX45, has a great sound and a relaxing handling.
Lotus Europa '71, interesting handling in my opinion.
Mazda RX-7/RX-8, using semi racing exhaust they make great noises, handle nice.
Lexus IS-F/LFA
Mercedes-Benz CL600, lots of flaws with that car and even more with it being a standard in GT5 (no cockpit, meh sound, meh looks), but it's still a big Benz limousine.
Nissan 370z
Pagani Zonda C12/C12S/C12S 7.3, amazing sound obviously, interesting to try various exhaust types on them too.
Spyker C8 Laviolette
Subaru Impreza/Legacy models - lots of standard versions so you probably have them sitting in your garage already, especially the Legacy makes a great low speed cruising car. Both are available both as sedans and wagons which is pretty rare in GT5.
TVR V8S, great sound, nice car, nice handling.
Just a quick write up from my mind, might have forgotten some great cruise cars. But you'll be busy with those for a while. 👍