It might be more accurate to say he was the weakest for purposes of the general. Weakness in that sense could not be measured in pure primary vote count, because it was a prediction of fitness for a future condition.
Another way to look at it is category rather than individual. Romney was the dominant liberal Republican, and that category of primary candidate never got the preponderance of primary voter support. As a category, the clear winner was the composite conservative candidate. For example, if Palin had been in the race, she would have likely zeroed out support for all the other conservative candidates, making it a straight up contest between her and Mitt, which I believe she would have won.