Oilers fan here and I'm going to say the Canucks.
And that's simply based on the size of the market. Metro Vancouver has nearly twice the population the Alberta capital region does. And then you have the rest of BC, which itself is larger than Alberta (ca. 5 vs 4 million residents) and doesn't split the fandom among 2 teams.
Now at least in my (personal and very annecdotal) experience, the Oilers have more of a following in other parts of the country - I lived in Winnipeg in the mid 2000s for instance and there were a ton of Oilers fans there before the Jets returned. Surely many switched allegiances since, but juding by Winnipeg home games vs Edmonton, many did not). Also, I'd be willing to bet that a larger percentage of Edmontonians are Oilers fans compared to the percentage of Vancouver residents who actively cheer for the Canucks. Still, the difference in population is too great. If Vancouver stays competitive for a few years, I think we'll see way more of those casuals / current non-hockey fans jump on the wagon of success and the popularity of the team will really baloon. The Oilers are maxed-out in their fandom. Vancouver has a room to expand into.