My favorite John Cena feuds include:
Kurt Angle (Smackdown early in his career - LOL @ the rap battles)
Edge (his career long nemesis they never have a bad re-feud)
Umaga (This series really made Umaga for me and made Cena good for a minute)
Chris Jericho (just cuz I love Jericho even though he was made to look awful
)
worst feud:
Big Show
Khali
Orton (way way way too long and their HIAC was miserably slow)