I don't think the Chiefs win over the Vikings in Super Bowl IV is really an upset, because history has shown that the Chiefs were most likely the better team.
I think in terms of talent disparities, the Steelers win over the Oilers in 1989 was a huge upset. The Steelers were one of the worst playoff teams I have ever seen, and the Oilers had been to the playoffs the past two seasons and were at home, but somehow Jerry Glanville bungled things up and the Steelers won 26-23 in OT.