Air: Simplest.
Rail: comfortable. Train station is a short shuttle-bus ride from YUL. It will save you 15-20 mins compared to driving.
Car: Relative easy, mostly highway drive from YUL to Ottawa. Check prices as there may be a one-way drop-off charge. Careful about fatigue after coming off a transatlantic flight. The highway is mostly boring and you wouldn't want to fall asleep, or miss the right turn to Ottawa and end up 5 hrs later in Toronto.
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13F