what team was playing the cubs on ferris buellers day off?

Because the Wrigley Field scenes were filmed on two different days - Atlanta Braves and Montreal Expos.

From the play-by-play descriptions, the uniforms, and the player numbers, this game has been identified as the June 5, 1985, game between the Atlanta Braves and the Chicago Cubs.

All these "in the park" shots, including the one from the previous scene where Ferris catches the foul ball on TV, were filmed on September 24, 1985 at a game between the Montreal Expos and the Cubs. During the 1985 season, the Braves and the Expos both wore powder blue uniforms during their road games. And so, with seamless editing by Hughes, it is difficult to distinguish that the game being seen and described in the pizza joint is not only a different game but also a different Cubs' opponent than the one filmed inside the stadium.

The Atlanta Braves.

Braves