As others have intimated, the question is, on a certain level, silly. Actors don't own roles (much as some of us like to think they do), and just because someone plays something first doesn't necessarily mean they do it best. That said, here are a few instances where I preferred a replacement over their original:
Estelle Parsons was 1,000 times better than Deanna Dunagan in AUGUST: OSAGE COUNTY. I know many others who felt similarly.
Bernadette was better than Catherine Zeta-Jones, hands down. (Bradley Dean was also an improvement on Aaron Lazar)
I preferred Laura Osnes to Kelli O'Hara in SOUTH PACIFIC.
Beth Leavel was far superior to Christine Ebersole in 42ND STREET.
In WICKED, I'd say that Murney, Bean, and Espinosa were better than Menzel; Hilty was far better than Chenoweth; and Hearn was a better Wizard than Grey.
And right now, I'd say Judith Light is better than Linda Lavin in OTHER DESERT CITIES.
"You travel alone because other people are only there to remind you how much that hook hurts that we all bit down on. Wait for that one day we can bite free and get back out there in space where we belong, sail back over water, over skies, into space, the hook finally out of our mouths and we wander back out there in space spawning to other planets never to return hurrah to earth and we'll look back and can't even see these lives here anymore. Only the taste of blood to remind us we ever existed. The earth is small. We're gone. We're dead. We're safe."
-John Guare, Landscape of the Body