Venice to Airport

what time of day, how many, where in Venice, international flight or within Europe?

These facts affect the answer

Define "Best". Quickest, Cheapest, Most Comfortable?

Just take a taxi from the plaza by the train station. It is not that far and will minimize walking to get into the terminal.

People define "best" in many ways. But taking a vaporetto to Piazzale Roma, and then a taxi or bus to the airport is one option. Or an Allilaguna boat from the stop nearest to your hotel to the airport (more expensive). Or a private water taxi from the nearest dock to your hotel (ask the hotel desk), direct to the airport (most expensive).

Our first trip to Venice was just a few weeks ago. We got the Vaporetto 72 hour pass with the 1 one way bus to the airport "add on". Once we got used to how the system works, it was easy and we would do it that way again. I believe it is the #5 bus from Piazzale Roma. Interestingly, we validated our tickets for every ride, and never got checked until the bus ride to VCE, and they checked us twice... So be sure to validate, you never know when they will target you!!!

See my reply on your other post re vaporetto passes.