I'm not sure any drink qualifies as "expensive" on AC flights, but here's my routine:
Domestic / transborder -> Canadian Club & 7-Up
International -> Crown Royal & Coke Zero
Plus some sort of wine with my meal and either Porto or Cognac with dessert. Which is strange, because I'm more of a whisky guy (as evidenced above) but I don't like any of the Scotches AC has on offer.
Enjoy your flight!