Depending on what time of day you arrive and how much luggage you have, there are 3 real options:
1. Subway.. Metro.. clean, fast, cost is 16 euro per person one way but you have to schlep your luggage around alot.
2. Taxi.. fast, no schlepping of luggage, will cost about 40-50 euros during the day, more at night.
3. Bus.. a little schlepping.. cost is 4.50 euro one way. express buses go frequently from airport to all central locations in Athens.