If you ride mainly paved roads a road tire will make a huge difference vs knobbies.
I've been running Specialized Fatboy 26"x1.25" slicks on my Marin mountain bike(rigid) since the late '90s. They are very smooth rolling and fast (at 80psi). The Flack Jacket protection works great against punctures. Great when its dry but not so good in rain or gravel.
If you're looking for more comfort go with a wider tire (1.75") with a tread. Don't go too cheap as they tend to be heavier and more prone to punctures.