I've said on many occasions here and elsewhere that Burton's performance in WHO'S AFRAID OF VIRGINIA WOOLF is for me the greatest performance ever not to have won an Oscar.
Least favorite Burton performances are EXORCIST II: THE HERETIC and THE MEDUSA TOUCH.
I love Taylor in A PLACE IN THE SUN, GIANT, VIRGINIA WOOLF, and CAT ON A HOT TIN ROOF.
I recently rewatched the second half of CLEOPATRA, the part I had always disdained, and I found it much more involving and Burton's performance much better than I remembered it being. I think Taylor's work is brittle and not very good throughout the film.