Can you give more detailed description about the ride quality you're looking for? "Smooth" to me has as much more to do with the road surface than the tire.
In general, if you are looking for a compliant ride, one where you feel the bumps less, you'll want to look for a supple tire. This is a tire which has a flexible rather than stiff sidewall and carcass. Usually the higher the thread count (TPI), the more flexible the tire will be. If you can find the TPI of the tires you like, you can use that as a baseline for comparing other tires.
I've had good experience with Panaracer tires. I've found them to be very compliant and comfortable without being too draggy or slow. Compass tires also have a good reputation. I understand they are made by the same company or at least designed by the same company. You'll also find Compass tires branded as Rene Herse.
The high TPI tires can also be a bit expensive so, you may want to shop around. 26" tires are usually pretty reasonable in price but high quality ones are definitely more expensive.