I wouldn't say Reservoir Dogs is perfect, but for what it tries to be, it is about as close as you can get.
I haven't seen the Hateful 8, but I assume the problem is that he tried to make it his Sergio Leone epic, without Leone's sense of style and storytelling. The Good, The Bad, and The Ugly is a story about 3 guys looking for the same stash of gold. There is no way it should be a ~3 hour long movie. But there are so many entertaining nuances and beautiful shots that it doesn't seem like it is as long as it is. There is no reason we need to see 3 minutes of Tuco running around the graveyard, or his and Blondie's detour in to the Civil War camp. But they add so much to their journey. And by the time you get to the final 3-way showdown, you are so wrapped up in that journey you have been on with these three guys, the climax is more than worth the wait.