to me there is nobody who is the "greatest" skater of all time...
the list skaters who have had the most influence on skateboarding and the direction of progression in the street skating era to me is as follows:
rodney mullen
tommy guerrero
natas kaupas
mark gonzales
mike vallelly
matt hensley
ed templeton
jason lee
eric koston
guy mariano
henry sanchez
daewon song
pat duffy
andrew reynolds
jamie thomas
chris cole
now this does not mean that i'm a big fan of all the dudes on this list. however, i do respect what all of them have brought to skateboarding.