For gravel rail to trail riding have been very happy with the 26 X 2.0 (559 X 50) Schwalbe Marathon Dureme on the front and Extreme on the rear. I think the Extreme is no longer made and it was quite noisy on pavement. Their Mondial may have replaced the Extreme, I have not seen a Mondial. I have not used the Dureme on both front and rear but I think that would work very well too, if I was going to do a trip that was half gravel and half pavement, I would consider the Dureme on both front and rear in 2.0 width.
On pavement I have been very happy with the 26 X 1.5 (559 X 40) Schwalbe Marathon (with Green Guard). Takes high pressure and rolls very well. The reason that I specify the Green Guard is that it helps you figure out which tire I used since there are so many in the marathon series. I used the 1.5 width at the time of this photo.
