RedSox,
Assuming you don't need to withdraw more than your daily limit, Bank ATM machines often give the best exchange rate. Most cards from the US should work, particularly for withdrawals against checking accounts, rather than savings accounts.
I would avoid Royal Bank and Desjardins, as they will both charge approximately $3 for the transaction. Banque National, Bank of Montreal and TD Canada Trust do not charge a fee when using a foreign card. I don't know about the others. Of course, you'll also probably pay a transaction fee to your bank. The exchange rate used should be within a penny or so of the official bank rate.
Respectfully,
Ned