I don't know why Buick engines were different from those of other GM products. According to this Wikipedia article, the biggest Buick V8 was a 455. But the cars that had 455-cubic-inch engines were such big boats, that they couldn't go very fast.
Pontiac also had a 455. Oldsmobile had a Rocket 455, but probably its most famous engine was the Rocket 4-4-0 (which was actually 400 cubic inches). The Rocket 88 and Rocket 98 numbers had nothing to do with engine displacement. At 572 cubic inches, it looks like Chevy had the biggest engine, but I gather that this engine was used in trucks. Looks to me like the Cadillac 500 was the biggest GM passenger car engine.
The 428 was the biggest gasoline-powered Ford engine I found. The Chrysler 426 Hemi was a famous engine. And hey, I once was issued a high-powered car as my government vehicle - an American Motors Matador with a 401 V8. It sure was an ugly car, but it had great acceleration.
As far as I can tell, the largest production passenger car engine is the Dodge Viper V10, 510 cubic inches. This page says GM built a 572-cubic-inch crate engine (you had to install it yourself...).