IMO...Certainteed is the best, not because they cover their roof warranties... which is a very subjective statement, but because they have kept the weight of their shingles intact. Their accessory cap has the same wind warranty as the shingle (GAF does not). A ignorant homeowner can get certified by GAF with a check, whereas if a homeowner wanted to get certified by CertainTeed he would have to learn something about roofing first and that something would make him more knowledgeable than 80% of the roofing contractors out there.
GAF was the first to offer lifetime warranty on the exact same shingle that last year was a 30yr warranty. I feel that makes a huge statement about the character of the company (not that I think Certainteed is any better, at least they are upfront about profit and money being their top priority). Since they made the change in warranty they have started making their shingles lighter... I guess lime was ok last year but not this year. You pay for a certification from them, no roofing knowledge needed, just a check. The Deckmate is an expense that no homeowner needs.
IKO use to be neck and neck with Owen Corning for worst shingle. However; in the last couple of years they have been making strong strides to improve their reputation and quality. Unlike GAF they too have kept the weight of their shingles and they are considerably less expensive than the other two and they do have the 15yr warranty.
All of this is again my opinion based on 23yrs as a roofer and salesman... let me finish by saying, last year we put CertainTeed on my aunt's roof, this year we put CertainTeed on my brother's roof and next year we will be putting CertainTeed on my parents roof... and we won't be using the Diamond Deck.
ps... if you get your roof put on by a Select ShingleMaster and he registers you for 25sq of the 5 star warranty; even if he does install the shingles upside down, using bubblegum to hold them in place... CertainTeed will warranty the roof for the 25sq.
However; since the contractor electronically signed when he registered the roof and warranty saying he installed the shingles according to their instructions, installed the proper amount of venting, I & W and replaced all the flashing... they will at a minimum boot him from the program and most likely go after him for damages.