John Kennedy, elected in 1960. The only other Senators ever to be elected directly to the Presidency were Warren Harding, elected in 1920, and James Garfield, elected in 1880.
Not one of them completed his first term in office.
My husband and I were trying to remember if GHWB was ever a Senator. The post most elected to the Presidency would be a former Governor. We think JFK was the last Senator, too.