Halladay is a must start. He is on something like a 8 game winning streak, and the Pirates are a pretty weak offensive team with a terrible pitching staff and bullpen. The Jays have a ridiculously potent offense, so he pretty much should win no matter how bad he pitches. The only concern that I'd have is that SkyDome is starting to act like an extreme hitters park this year, while it's been pretty neutral in the past.
I would also start Pineiro. It's at home, in a pitcher's park; Pineiro is a quality pitcher and Montreal is playing without Vlad, so they don't exactly rank as a scary offense. Willis should be a decent start, but I wouldn't fully trust him. Rookie pitcher are a constant risk of getting lit up, although the Brewers are a pretty weak team, so I'd probably chance it. I would personally get rid of Willis in a non-keeper as soon as he has two straight bad starts, because he is pitching way over his head right now and I seriously doubt he's the second coming of Kerry Wood.
I would leave Buerhle on the bench for this one. Although he has been coming around, US Cell is a hitter's park, and Bonds and co. have the ability to rake him over the coals if he's even a bit off. If he does well in this start against the strong Giants offense, it's probably a sign that it's safe to use him most of the time again.