I'm 18, and I confess I've never seen a straight play.
However, for most memorable performance in a musical (speaking strictly of what I've seen live), I'd say Carl Anderson as Judas in
Jesus Christ Superstar. I'd seen him in the film and expected him to be good, but I also thought maybe his age would hinder his performance. Man, was I wrong.
I have never seen someone with so much energy on a stage in my life. It was a commanding performance, and he carried himself with a sort of dignified passion. Watching him destroy himself as Judas was like seeing a man set himself on fire before your eyes in terms of how intense this guy was. His voice was beautifully musical, and in a sense it was muscular.
His performance was the closest to actual method acting that I think I'll ever see. He put so much emotion into his performance that it scared me (especially during the Last Supper, where his delivery of "Cut out the dramatics / You know very well who" left me with a strange, eerie deja vu feeling). I'll never forget the moments before Judas committed suicide, seeing Carl staring into the spotlight with tears flowing from only one eye as he sang, "When he's cold and dead, will he let me be? / Does he love me too? / Does he care for me?"
Met him at the stage door after the show when I was seeking an autograph, and if it's ever said that anyone has given 110% for a show, it would apply to Carl. He literally had "no voice" when it was over. A few months later, he was gone. This performance remains one of my fondest theatre memories so far.
"There is no problem so big that it cannot be run away from."
~ Charles M. Schulz
I mean, I'd still pick Slinger over him, but he was still so unbelievably amazing that I'll spend the rest of my life chasing Moors only to find them all falling short. 
