#1 Becoming Grand Slam Champion
Punk missed out slightly on becoming a grand slam champion in the WWE. Had he won the US title once, he could have etched his name in the list of Grand Slam Champions. Punk won three of the World titles, and also the Tag team and the Intercontinental Championship.
However, he failed to meet the criteria to become the Grand Slam Champion, as he held only one of the two secondary Championships in the WWE.
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If CM Punk would have been a Grand Slam Champion, he could have cast an unbelievable legacy in the WWE. He was by far the most controversial Champion that WWE has ever seen.
If WWE could have channelized his potential in the right direction, he could have had many more accomplishments on his resume. This accomplishments are not Punk's failures, rather a reflection of poor booking on the part of the WWE management.