I'd definetly have to say flames and oilers.
need I remind you of craig mctavish ripping the toungue out of the calgary mascots mouth?
but really.. it's not just the flames and oilers, it's actually everything calgary and edmonton, hockey, football, any uni sport.
basically there two very similar sizerd city's, ones where the government is, the other is where the business is, ones liberal, ones conservative, etc. basically if edmontons playing, you cheer for whoever else is playing them, and visa versa.

Dodgers-Giants in Baseball

Cal-Stanford in all College sports

Manchester United- Liverpool in English Football

How about some european soccer (like Man U-Liverpool which the Duke fan above me said)

Real Madrid-Barcelona

AC Milan-Inter Milan

Rangers-Celtic

I wonder how big these are?