I'd say LeRoy Butler.
And I think the only position better than any other to have your best player at (excluding special teams) is QB. That's the only "rule", IMO. HBs usually need lines. WRs need lines or QBs. One lineman does not make a solid unit.
D-Linemen need other D-Linemen so they can not be double teamed. Linebackers need other linebackers for both pass coverage and run coverage, and also need solid linemen in front of them to take on blockers. DBs need a decent D-Line so they aren't covering their guys for 5 seconds straight.
QBs, although they cannot make a unit, make the largest impact on the game. And they can do without a good offensive line sometimes, or a bunch of good receivers.
Really I don't believe there is a rule about winning championships, but if there was one the first position I need filled on my prototype championship team is QB.
If there was a rule.
And just to clarify, I don't believe Marshall Faulk was necessarily the only RB who was best on his team, I only want to point out that it happened in the last 22 years, making it no more unlikely to happen than the average. There may or may not be other examples; I didn't even look into it.