While his win against JBL was nice and all, it was a terrible match. Yes, it was for his first WWE Championship, but the match was so bad that it wouldn't have been good as the main event of Raw, let alone the co-main event of Wrestlemania.
An incredibly important match for Cena was his WWE Championship match at Summerslam 2005. It was truly when the "Let's Go Cena, Cena sucks" chants began to emerge, and he began to be seen as a 'controversial champion', instead of him being cheered as a face and Jericho the heel. It also was his first one-on-one WWE title defense while on Raw, and he did so successfully against the first Undisputed WWE Champion during a time that Jericho was quite relevant. It wasn't well known that Jericho was leaving the company the following night, so it was a statement match for Cena, and he delivered with a very good performance.
Still, I think his most important match came at Wrestlemania 22, against HHH. For three reasons.
1. It was Cena's first Wrestlemania main event, and he won.
2. He made HHH, a ten-time former champion, tap out, clean as a whistle.
3. It was a very good match, in the ***1/2 to ***3/4 range.
It wasn't a classic, by any means, but it was a solid main event that showed that not only could Cena hang with HHH, but he was going to be the 'guy'. He was in the main event of Wrestlemania, and he made the man who was easily the top heel at the time in HHH, tap out. It showed WWE's faith in Cena, and he has paid them back in spades.
It makes me wonder, however, what would have happened had he had a bad match. That would have been two bad Wrestlemania matches in two years, and the trajectory of his push could have changed altogether. Instead, he had a very good one, and it lead, I believe, to him being where he is now. A 13-time WWE Champion. Had he failed, who knows what would have happened. Hence, it was his most important match.