For me it would be a tie. Although I only rode it once, the Silver Sky, as a Rio Grande Zephyr car, was one of the best train rides I've ever had, though due partly to the circumstances. I was fortunate to ride The Last RGZ to make the entire run from Denver to Salt Lake City. After taking the siding East of Thistle for an E/B coal empties, my then - girlfriend and I watched from the Silver Sky dome as we tip - toed through the Thistle mudslide. At this point it was only a bulge pushing the track up from underneath, making for an E - Ticket ride as we watched each car of the train rise & fall over the bulge. The return trip the next day through wonderful Wyoming, at a speed - restricted 55 MPH thanks to a paranoid EwePee, was forgettable.
My other favorite Obs car would be the old Native Son, the former Union Pacific # 9006 Dome - Lounge - Obs. I was fortunate to have ridden this car several times on both deadhead moves and railfan charters out of Los Angeles. Cruising @ 79 MPH on the San Diegan run, and riding up the Coast Line to Oakland, were some of my best memories as a young railfan.