The Riviera was interesting within GM.

For the first-gen, it really spoke to GM’s faith in the model that it was allowed to have its own platform for the first generation, 1963-1965. That almost never happened with anything that could reasonably be shoehorned onto a common corporate platform.

I agree with you on the gen. 2. Beautiful in 1966, hideous by 1970. What’s more, it was decreed that the gen. 2 Riviera would share its redesigned E-body platform with the Eldorado and new Toronado, only with a different layout. The Eldorado and Toronado had partial frames and longitude-FWD, while the Riviera got a full frame and traditional longitude-RWD. Not only that, the Riviera used the controversial cruciform-frame (X-frame) design that GM had otherwise retired after 1964.

As for the boat-tail, which was the gen. 3 (1971-1973) Buick designers wanted to move the Riviera to the A-body, which would have yielded a much sleeker car. But GM management either thought the A-body wasn’t premium enough or had other reservations, and decreed that the Riviera would need to remain an E-body. Both the B-body and E-body cars received full redesigns for 1971 and were enlarged majorly, and really there wasn’t much difference between the B and E at that point. Thus, the Riviera, which kept its RWD layout, was basically a B-body and therefore a stylized variant of the LeSabre. It even shared door glass and a windshield with the LeSabre. So it was a B-body masquerading as an E. Ditto the 1974-1976, which used the same bones as the prior one.

GM successfully downsized the B-body cars for 1977. At that point, the Riviera—which was also redesigned—was officially designated a B-body, and was again essentially a LeSabre in drag. But it was just a holdover model, lasting from 1977-1978.

The E-body cars got downsized for 1979, and the Riviera joined them. This time, it finally adopted the longitude-FWD powertrain setup that the Eldorado and Toronado had been using that whole time, for what would be that layout’s last hurrah. It also got a four-door cousin, the gen. 2 Seville (designated a K-body, but entirely an E-body). That generation lasted through 1985.

The 1986 downsizing saw all of the E-bodies and the K-body Seville go transverse-FWD, and wasn’t very successful. The Riviera’s controversial touchscreen wasn’t much help, either. It did get a smaller relative, the Reatta, essentially a sawn-off, 2-seat Riviera under the skin. Both were facelifted, successfully, in 1990. The seventh-gen lasted through 1993.

The Riviera skipped the 1994 model year and the eighth-gen bowed for 1995, sporting dramatically new and curvy styling, and a G-body platform shared with the new Aurora. Engines were either the base naturally aspirated 3800 or a supercharged variant. The 1995 Riviera was an odd one-year model with the older Series I base engine, older electronics, and the OBD 1.5 standard. In 1998, GM made the supercharged engine standard. Unfortunately, what the Riviera didn’t have was the Aurora’s attention to detail or materials…or that car’s Northstar-derived V8. That, combined with the decline of the PLC market in general, meant that the Riviera did not survive into the new millennium and was canceled after 1999. A final run of 200 special Silver Arrow Rivieras commemorated the model.