Hmm...maybe not "best" but two memorable ones that come to mind:
TUCK EVERLASTING: I'm probably the only one who had a best time there! LOL Kidding...I found the show charming. Anyway, what made it best was I was seated next to a stranger, a young woman studying scenic design at NYU-Tisch. I'm a major set design junkie, oogling the sets is often my favorite part of seeing a show. At intermission I grilled her on elements in the show, as well as other recent shows I'd seen to find out how the designers and crew did things. It was a fascinating, educational, and enlightening 20 minutes for me.
PHANTOM: THE VEGAS SPECTACULAR: My friends know I am anal retentive about getting to shows well ahead of curtain so I can relax, read the playbill, and soak in all the pre-show excitement and buzz. My dear friend, Michael, and I had tix to Phantom and, the next evening, O. I got the curtain times reversed -- POTO was 7pm, O was 8pm. Fortuately, we had gotten to POTO early to get tix at Will Call and have a cocktail. At 10 minutes to 7 I found it odd the doors were open and so many patrons were rushing to their seats for an 8pm show. I gasped when I realized my mistake. We were very nearly the last two escorted to our seats and the show began seconds later. I loved the production, but it was memorable because my dear friend would pass away a year later, and it was the last of many musicals we saw together.