OK, nobody has an opinion so I guess I'll just talk to myself. I'll limit our list to Las Vegas proper. Hmmm...
If we go backwards it's safe to say that Palazzo and Wynn are both in their original buildings.
Venetian and Mandalay Bay have both expanded but still have their original buildings so do we throw out expansions as long as the original building is still there? Let's go farther back...
Luxor has expanded and is in its original building, sort of, it has remodeled and changed so much it is nowhere near it's original state.
Hmmm... so if the "shell" of the building is still original, is it still considered the original building? I have to ponder that.
Let's look at it from the other direction. If a building was originally built as a casino and kept the name, which casinos still remind you of the original that was built. Who remembers the Riviera when it was originally built? Is there anything left from that original building other than the walls? How about the Sahara, Tropicana and Caesars? Is there anything left that makes you think of the original? I assume the El Cortez is still mostly original but once we get past the El Cortez everything gets hazy. Does "anyone" have any opinions?
How about you Fred, you started this!