I would say it's pennies, with nickels a close second.
My hunting has been almost exclusively slot machines since the mid-80s and in that time I've found coins from 16 different countries besides Canada.
Penny machines have produced coins from nine foreign places: Austria (2 euro cents), the Bahamas (cent), Barbados (cent), Bermuda (cent), Germany (2 pfennig), Guatemala (centavo), Jamaica (5 cents), Mexico (5 centavos) & Panama (centesimo).
Nickels have produced six: Aruba (casino token), Belgium (franc), Ireland (10 pence), Saudi Arabia (10 halala), Singapore (20 cents) & Switzerland (10 rappen).
Halves have only produced one so far, an Australian 20 cents.
Considering slot machines' sophisticated mechanisms, it's amazing foreign coins can still fool them.