For me, Brewers. The peak era of Cubs success began in 2015 and since 2018, the Brewers are constant pests with overlapping success. The Cubs just sucked during the peak of the cardinals rivalry and was rooted in tradition more than the Cardinals being an October roadblock to anywhere near the degree of Milwaukee.
It was also easier to rationalize the Cardinals success with players like Pujols, Rollen, Edmonds, Wainwright etc than a team that pumps out 90+ wins every season with an 82 win roster on paper, adding to the frustration. That and the Packers connection.