Tyne Daly in Gypsy - her Rose's Turn was an acting feat
Marie Mullin in Beauty Queen of Leenane
Sutton Foster in Anything Goes - she was some sort of miracle on stage. Couldn't take your eyes off her.
Daveed Diggs as Thomas Jefferson
Jonathan Groff as Claude in Hair, in Central Park on a magic night
And I also want to mention performances by people in small theaters long forgotten - whoever played Pseudolus in a 1970-something performance of Forum by Florida State University on Jekyll Island, Georgia; whoever played Sandy in a Prime of Miss Jean Brodie somewhere in South Florida in the 70's at some point... these and other shows and performances stick with me, showing that great theater can take place anywhere.