I think along a different lines. Yes being able to dominate in the your speciality is one thing, bu what I take into account is your competitiveness all year round. Cavendish is the best sprinter in the world, but how many times do we see him not contest a sprint because he got caught out by a hill. In the last 2 years, outside of the Grand Tours, he has about 6 or 7 wins. For a sprinter that is not a lot. He has been the best at his specialization, but I haven't seen anything special that makes me go that is the best cyclist.
I watch Cancellara in the E3 come back from a 2 minute gap, by himself pass an entire field, and win easily. Eventhough he didn't win either cobbled classic, he was single handily the best in the field and the one who tried to win. I remember that flat stage in 07, where Canc single handly leapt out the peleton, cross to the break and held on to win the stage in front of the sprinters.
It was an uphill finish, but how many people thought he could win Fleche Wallone, the general consensus was it was too steep and long for him, and not only did he win it, but I could have landed a B-52 bomber in between him and J-Rod. Every race he enters, he has the possibility to win. He has finished on the podium in back to back years at T of Flanders.
I think Gilbert is a throwback to the times whenracers always wanted to win when they pinned on a number, not to ride for the purpose of training, maybe test themselves a stage or 2. Obviously Gilbert will not win a Grand Tour, but Ithink he could win a Paris-Nice, especially if he focuses on winning ToF next year since he got his Ardennes victory this year. Paris-Nice would be an excellent build up for him.