Unquestionably, for nostalgic reasons, for me it would have to be the Flamingo Las Vegas (formerly The Fabulous Flamingo and Flamingo Hilton Las Vegas). Not only because of its colorful history (oldest resort on the Strip, last remaining casino on the strip that opened before 1950 that is still in operation, Bugsy Siegel, Meyer Lansky, etc.) but also because my daughter was conceived there.
Good times.