Zep all the way. If you really wanna look at it realisticly, Zep was influenced by old blues players. If Zep wasn't the greatest of bands then why did How the West Was Won debut at number 1 the very day it came out? This after not releasing anything in 21 years (Coda and not the boxed sets or the BBC Sessions) and not any of it was original? More present day bands have cited Zeppelin as a direct influence over the kind of music they play than any other band that ever was.
Now I like all 3 bands, but the Beatles did start the British Invasion here in the states, but that doesn't neceessarily mean that they are the greatest. Floyd has put out some really great tunes (from possilbly drug induced times to sober days) that could tell a story. Zeppelin always fed on their raw energy which was most seen in their concerts. No other band has been able to duplicate the quiet acoustic setting of Earls Court and then turn right around and bust out the jams full tilt all in the same show.
Like was said previously, lack of radio airplay didn't harm Zeppelin one bit. They were enough to be able to both sell lots of albums and sell out their shows.