I will be dropping off a rental car in Venice and would like to know which drop off place is the most accessible by car: the airport or the train station?
Neither.
Your drop off location is called PIAZZALE ROMA.
That is where all the parking structures are.
The parking garage you want is the AUTORIMESSA COMUNALE (Municipal Garage), that is where the car rental returns are.
Below is the image of that Municipal multistory garage as you arrive from the mainland through the Liberty Bridge (Ponte della Libertà).
The rental offices are on the ground level (you can see them in the image). Ask the office where you should leave the car (inside or outside).
https://goo.gl/images/yXMXwL
Follow directions to Piazzale Roma until you see the parking garage.
Roberto's answer assumes that you are staying in Venice. At drop-off you would be wise to photograph the dashboard showing the odometer and the gas tank level - presumably full. Also photograph around the car showing the lack of damage. Mind the notorious speed cameras on the causeway.
Many thanks, Roberto, for your very helpful advice.