Tokyo_Bunny_8990: Bethesda (a studio that at best makes games that "eventually" become good).
Worth noting that Bethesda games, or at least main series TES games, have generally improved in terms of bug density over the years.
Arena has fewer bugs because it's simpler, but it still manages to have quite a few bugs lurking about, some of which are just embarassing. Like being able to easily levitate out-of-bounds in a town, causing the game to hardlock.
Daggerfall is buggier than Arena, but that can be explained by the game being more complex, and hence a much bigger bug surface. (For example, Arena's mostly 2d gameplay meant that falling through the floor and into the void would not happen, but Daggerfall being fully 3d allows that possibility. Also, the new skill system is a ripe source of bugs.)
Morrowind isn't as buggy as Daggerfall. There's still falling through the floor (except that the game will teleport you to a reasonable spot in the room when that happens), and I've had the game crash when getting a Cliff Racer to fall from up high into a city, but not as bad as Daggerfall. Also, the game handles the situation of no playable races, something not possible without modding, without crashing.
Oblivion, in turn, isn't as buggy as Morrowind.
I don't think I even had Skyrim even crash once. (Worth noting that disabling the player via the console, which would crash Oblivion, isn't even allowed in Skyrim.) I haven't fallen through the floor, either. An NPC did once fall through the floor when I wasn't here, but after using the console to teleoprt her to where she should be, her AI script started working properly again.