Best located airports (Barcelona, Rome and Venice)

Rome, FCO and CIA are about equal. Venice, VCE is MUCH MUCH closer than Treviso. Treviso is basically another city while VCE is a short vaporetto ride away from downtown Venezia.

BCN's the main Barcelona airport, a 30-minute bus ride to Plaza Catalunya....service every 15 minutes aboard the Aerobus, 3.75 Euro. Barcelona-Girona is about 65 miles northeast of Barcelona...bus service coordinated with the Ryanair schedule takes about 1:15 to make the run, cost 12 Euro. Buses leave as soon as they fill up, so that may add to the commute time. Barcelona-Reus is about 70 miles southwest, again with bus service coordinated with Ryanair arrivals and departures. Same procedures as above, with one way fare at 11 Euro for a 1:30 run.

Obviously a way out-of-date follow-up to the question and responses.
Between Marco Polo and Treviso the better choice is, in my opinion, Treviso.
Reasons:
The airport is very small--quick recovery of luggage.
Better, cheaper connection to Venice--walk 10 meters out the door, board the bus (~3E) to the train station, take the train to Venice station(~4E).

Compare this to a larger, more expensive VCE and longer, less comfortable, more expensive trip via vaporetto. Note: the one bathroom on the boat isn't always operational or available.

I'm not sure about Barcelona, although when I have looked it seems like the main airport, BCN, is much closer and more convenient than the much more remote airports used by cheapo airlines.

As for Rome, Fiumicino vs. Ciampino, Fiumicino has direct, regular, high speed train service into Rome, while Ciampino has only periodic buses.

Worst of all, in my opinion, is Treviso, which has only sporadic bus service, taking over an hour, IF you catch the bus that leaves right after the Ryanair flight arrives. If you miss that connection, you might be there a long time.

Lest someone get the wrong impression about VCE, let me pipe in. While VCE is larger than it used to be, it is still small compared to many airports. It is easy to navigate and the bus to Venice stops about 10 meters from the front door.

There are two buses that go to Piazzale Rome. One is the blue ATVO bus that costs 3 euros and is a express taking about 20 minutes to get to P. Roma.

The other bus is the orange ACTV bus which is a local and takes about 25 minutes to ge there. I think the ACTV bus is 2 euros, but it is also covered by both the Venice card and the vaporetto passes.

Treviso is about a hour's bus ride away.

The ACTV local bus is the best value going in Venice! I highly recommend it. I think it is €1.10.

It is just steps outside of the main entrance and before you know it you are in Venice.

You can buy tickets inside Baggage Claim from the machine.