Which tire has the least rolling resistance?

You need to consider a few things to get the least rolling resistance.

1) narrower tires generally have less resistance.

2) harder rubber compounds have less resistance.

3) higher air pressure generally will decrease rolling resistance.

4) tire thread count and psi ratings.

Now comes the tricky part, all the above factors must be considered as a narrow tire with a sticky rubber could have a higher rolling resistance than a wider harder rubber compound tire. There are trade offs with every tire design so select carefully and pick the one that suits the type of riding you do and where.

EDIT: "a tire with little or no tread pattern will roll easier than one with an agressive one, is that correct?"

A tire with as little tread (smooth/slick) and min. contact patch on the ground will 99% of the time have the least resistance.

For a tire that lasts a very long time, has a thick tread, rolls easy and is fairly cheap, check on Serfas tires. For a 26" rim size you should be able to find them for under $25ea at most bike shops.

The thinnest tires are the fastest, but you'll also notice the fastest tires have no tread.

Least