I use the Michelin LTX M/S myself since it seems to be the best compromise of a very low rolling resistance highway design but is still tough enough and gives great traction for offroad and severe weather. It also resists punctures extremely well and rides like a luxury car tire. Though it is the best tire I found for these purposes, I'm not so sure it's the BEST tire for mileage alone, since they do weigh a bit more than most other tires, and Michelin seems to make them in a slightly smaller diameter/circumference for the given size, but I do think it's the best compromise in a Truck/SUV tire for mileage, ride quality, and load carrying ability. The P-rated LTX M/S are much lighter than the LT-rated, but the P-rated are also a slightly smaller diameter.
I'd say right now, the best mileage in a practical tire would come from the new generation of higher mileage tires...something like the Goodyear Fortera TripleTred, which seems to be a lighter, truer-to-size, pavement-specific tread.
EDIT: I've done some additional checking on this, and it appears for the 255/70R16 size, the Michelin Cross Terrain SUV has a treadwear rating of 700 and a weight of 36 lbs, with the Goodyear Fortera TripleTred having a treadwear rating of 600 and being 37 lbs. Perhaps the Michelin is better, going by specs. The P-rated LTX M/S gets a treadwear of just 500 though, which is rather surprising since I know for a fact it lasts well beyond the 60-65,000 mile rating of the 600-700 treadwear tires.