What is the most cost effective and efficient way to travel into the city from the airport?

What is the most cost effective and efficient way to get from Lisbon airport into city?

I took the metro to get from Lisbon's airport to Rossio in the center. It did require one change of trains, but otherwise it was easy. There may be a bus that costs not much more that is more direct.

I took a taxi and it was only about $20usd.
There were two of us so it was economical

Taxi is very convenient - we took one from the airport into center of Lisbon and it was 12 euros including tips. It took about 20-25 minutes during morning rush hour.

On our way back, we took the metro from Baixa-Chiado station to the airport. It took 1 hour door-to-door.

If you want to be frugal, you can take the metro, which has a stop right at the airport (terminus of the Red Line). To get into the center of city, most likely you'd need to change at Alameda for the Green Line. The interchange has quite a bit of walking. Most subway stations have escalators or elevators but not all, and even the ones with them, you may still need to lug your luggage up/down a flight of stairs.