Growing up in the '70's in Boston, I went to a lot of great concerts at the Garden. Its really tough to say which was "the best," although I have to say that ELP's Brain Salad Surgery tour in December 1973 was a standout, as was Crosby, Stills Nash & Young at the Garden in 1976, which I thought was difficult to beat.
Shakti at the Berklee Performance Center in 1977 was transformational. Put me on another planet! But the most fun show was The B-52's at the Paradise Theater. Maybe 5 ft from the stage, dead center, sitting at the "record company" table with Steve Tyler and Rick Ocasek, it was just stupendous and everyone was dancing on the tables. I have a bootleg of that show and brings a smile to my face every time I listen to it! They were just phenomenal.