This one is more of a brief feud than a full-blown rivalry, but it must be on the list. Once John worked his way through Smackdown and became a WWE Champion, he was drafted to Raw and made legitimate by guys like Triple H and Chris Jericho. But there was one legend – one truly iconic name – that stood in the way to being considered truly great: the "Heart Break Kid" Shawn Michaels.
John Cena faced and defeated Michaels in the main event of WrestleMania 23 to successfully defend the WWE Championship and then, in what may be the single biggest legitimizing force in his career, the two went a FULL HOUR on Monday Night Raw.
#7 – Brock Lesnar
Brock Lesnar was the WWE Champion when John Cena broke onto the Thursday night scene – yes, Smackdown was on Thursdays – and the two quickly got in each other's way. It was the first major feud Cena had, and although he came up short, facing a monster like Brock Lesnar so soon after debuting with the company placed him on everyone's radar. It was also during that program that he came up with the Attitude Adjustment – then called the "FU" as play off the "F5" – which of course would become the move that Cena became known for over his entire decade of dominance.
In 2012, Lesnar returned to the WWE and made it his first order of business to march down to the ring, hoist up John Cena, and dump him unceremoniously on his ass. The two would go on to have a brutal, bloody, fantastic match at WWE Extreme Rules – a show in Chicago that I attended – which held its own on one of the greatest wrestling cards in WWE PPV history.