Oblivion was a great experience bug wise for me it had really no bugs of note except before any patches the game crashed a lot. Oh yeah and the patches for Oblivion actually patched up parts of the game without introducing game breaking bugs.
Fallout 3 was very buggy ash piles everywhere and then enemies gear respawning in the pile and the enemy never came back ever. The game crashed all the time there was an odd glitch where the sun would turn into blinding geometric shapes and hurt my eyes and so on.
New Vegas was made by Obsidion but it had even more bugs than Fallout 3 but it used the same game engine. It had the ash pile problem I mentioned above. It had very frequent crashes, the AI did idiotic things all the time screen tearing. Very annoying indeed.
Then we have Skyrim the worst bugged one of them all. Plants do not respawn ever in several locations all over the place. 1.6 failed to fully address the enemy respawns in the wilderness the wolves, bears, sabrecats and the like will simply not cone back out and about. Ash piles was not properly fixed with 1.4 very rarely some ash piles will never clean up. The game crashes but not as frequently as NV. Weapons and shields do not clean up properly from the game world. These are just my most bothersome bugs that effect me the worst and detract from the experience.
There will be those that defend this game to no end but that does not change the fact several things in this game are broken still six patches in. This game still needs work the bugs above need to be addressed for a good experience.