Off the top of my head, these are the 3 oldest left on the Strip...I think

Trop opened in 1957
Caesars Palace 1966
Circus Circus 1968

Downtown is the El Cortez (1941 I think), Horseshoe (1951), Golden Gate (1956-ish), Fremont (1956-ish), Four Queens (1964-ish). Still in the same Buildings...I don't think any are still owned by the original owners (or their descendants)...the Horseshoe, of course, has had a few name variations.

Then there's the Railroad Pass...pre-1931-ish?...but I don't know if any of the original building is still standing.

Don't quote me on these dates...I didn't look up any except the ones on the Strip!!!