Which is the best option for getting from Venizelos Airport in Athens to Pireaus to catch a ferry nowadays ?

Get the X96 bus which is just outside the airport exit doors and get off at the terminus, then a short walk to the dock gates across the road. Its the easiest and cheapest.

Take the metro from the airport and change to monastiraki to take the train. The train's terminal is pireas so when you change to train make sure you go "to Piraeus ". When you reach Piraeus you have to walk though to catch your ferry but just a few to go to your gate. There are signs almost everywhere out of the station. Or you can take the bus from the airport specifically the X96 bus which will take you to pireaus but then you will have to walk to the port, I don't know if it stops somewhere near the port. Hope I helped

There are two ways to get there. a). Bus, that will leave you a bit far off the port but within walking distance. b). Train with a couple of changes that will leave you opposite the port.

The Proastiakos train has a connection at Plakentias and then another one in Monastiraki.