The narrowest and highest pressure 26" slick I know of is the Hutchinson Top Slick,26x1, 130 psi.The Ritchey Tom Slick Pro is in the same category. The Michelin Wild n Run'R Light is very close to this 26x1.1 as is the Conti GatorSkin,26x1.125.
Any of these will give you a road bike like performance.
The next range is the 1.25-1.4 or 1.5",there are tons of choices here. We have a local criterium that includes a category for mountain bikes.They limit tire size to a minimum of at least 26x1.25 and no rigid forks, I race it on a Kenda Kwest which has a slight tread pattern, can be pumped up to 100 psi, and is available in 1.25 and 1.5 widths.
Tires bigger than this roll like pillows but provide a great deal of comfort and some off road capability.
I would spring for a kevlar beaded tire and remember that the lower profile tire combined with longer mountain bike cranks can lead to the pedal hitting the pavement if you corner with it down on the inside.