The bartender on the USS Enterprise-D is Guinan.
Guinan runs the Ten-Forward lounge, the ship’s social gathering place, in Star Trek: The Next Generation. Played by Whoopi Goldberg, she is more than a bartender: she acts as a trusted adviser and informal counselor, especially for Captain Jean-Luc Picard and the rest of the senior crew.
Guinan is an El-Aurian, a species known as “listeners,” and her unusual age and perspective give her insight into events that most humans cannot understand. Her history includes surviving the destruction of her people’s world and being rescued from the Nexus in Star Trek Generations.
The question sometimes causes confusion because Ten Forward is the name of the lounge, not the bartender. Quark is the bartender at Deep Space Nine’s bar, while Neelix is associated with Voyager’s mess hall. Guinan’s name also draws inspiration from Texas Guinan, a real Prohibition-era entertainer and club owner.