I'd say definitely Essen. I don't know if there were several shows in that city in '70, but there's one outta there that sounds great. Both in terms of sound quality and performance. Too bad there's not more boots of the same fidelity from that tour. Second after Essen, the best is probably Paris or one of the Frankfurt boots.
Paris 9/23/70 ("Shake Your Hips" version)- the closest thing to a soundboard from the '70 tour in circulation. It was a radio broadcast but, the way I understand it, it was recorded with an external mic held to the speaker. I'm sure somebody else could explain it much better, but it's probably my favorite recording from this tour.
Essen 10/7/70- very good audience recording.
Berlin 9/16/70 and Cologne 9/18/70 are both good audience recordings.
......and there's the other Paris "soundboard". Recorded in the same manner as the other Paris broadcast, the 9/22/70 show has similar sound quality but is marred by a French DJ who WILL NOT SHUT UP...he talks between songs, during the intros, sometimes in the middle of songs....