They say you always remember your first, right? In the case of CM Punk, his first taste of championship gold in WWE was when he won the ECW Title by defeating his rival John Morrison.
It was billed as a "Last Chance" Championship match, which meant that if Punk failed to win then he would not get another opportunity at Morrison's title. It was a huge win for the babyface Punk, who captured the championship after 15 minutes of excellent action thanks to his patented Go To Sleep (GTS) finishing move.
The crowd reaction to the match was terrific. There are a lot of bad things you can say about WWE's ECW brand because of how it was booked, how talent was pushed and how it faded away into dust within a few years.
My opinion is that Punk vs. Morrison in 2007 was the best feud under WWE's ECW umbrella. It is this win by Punk that drew the biggest crowd reaction and made people care about that Championship. They worked hard, they had awesome chemistry together and CM Punk left the match as a conquering hero that won over a lot of fans who believed in him long before he set foot in a WWE ring.
While the ECW Championship isn't considered as a major championship, it's still important to remember how important it was when Punk won his first title in WWE.