Hello We are renting a car in Sondrio and driving to Ortisei for a few days then onto Venice. I'm looking for recommendations as to where to drop the rental? Thanks
What company?
Yes it matters which rental agency.
If you are spending time in Venice you do not need or want to go to the airport to drop car so it would be Piazzale Roma or Venezia Mestre.
From Mestre you would take the quick train into Venice proper.
From P Roma you can walk or take a vaporetto to wherever you are staying
The choice should be dictated by where the hotel in Venice is.
If you plan to stay at the Venice airport to catch a flight the next morning, then return it at the airport.
If you plan to stay in Mestre, then return it in Mestre.
If you plan to stay in the historical center of Venice, in the middle of the lagoon, then it makes sense to return it at Piazzale Roma.
Most major companies have offices at all of the above locations, although I think Avis-Budget has closed the Piazzale Roma location.
Be aware that on Saturday afternoon and Sunday all day, only the airport locations may be open.