Well, from the games I' ve played:
Elder Scrolls: Arena
Elder Scrolls II: Daggerfall
Elder Scrolls III: Morrowind
Elder Scrolls IV: Oblivion
I can say that no games can come close to the size and complexity of TES.
Arena and Daggerfall were unique and groundbreaking for their times because of their massive scale. Arena had some great randomized dungeons far before the likes of Diablo and is much bigger than Morrowind and Oblivion. Quite a feat back in 1994.
Daggerfall was truly epic in every sense. I' ll quote what wikipedia says about the size:
Daggerfall is the largest Elder Scrolls game to date, featuring a game world estimated as being 161,600 square miles — roughly twice the size of Great Britain — with over 15,000 towns, cities, villages, and dungeons for the player' s character to explore. According to Todd Howard, Elder Scrolls programmer, the game' s sequel, The Elder Scrolls III: Morrowind is 0.01 percent the size of Daggerfall. Vvardenfell, the explorable part of the province of Morrowind in the third game has 6 square miles. The Elder Scrolls IV: Oblivion has approximately 16 square miles to explore. In Daggerfall, there are 750,000+ non-player characters (NPCs) for the player to interact with, compared to the count of around 1000 NPCs found in Morrowind and Oblivion.
While not as detailed as Morrowind/Oblivion, if that' s not a technological feat, I don' t know what is.
Morrowind was great simply because of it' s unprecedented detail. It had some amazing graphics and was still massive compared to every other game out there. Every part of the game was handcrafted and specifically placed. Considering the amount of places you could go, this really does amount to something impressive.
Oblivion pushed the Morrowind concept even further with completely amazing visuals, sprawling forests, massive render distances, highly advanced AI.
Radiant AI was their AI system that you' ve probably heard of. Every NPC has needs and have to fulfill those needs based on their individual stats. Sounds simple but is very complex and adds a lot of realism to NPC actions. This was developed fully in house by Bethesda and one of the most ambitious AI projects to date. While not as successful as they wanted (in order to fix problems, Bethesda had to cut down the AI system to a more basic version) I still think it deserves praise.
The other parts of Oblivion weren' t developed in house by Bethesda but were modified to suit Oblivion' s needs. SpeedTree, Havok, FaceGen, Gamebryo were all incorporated into Oblivion to create a super immersive world. But you must note that most of these same technologies are being used in UE3, and a number of other modern game engines. Once again, all aspects of Oblivion were hand crafted and specifically placed, right down to the last book.
While my explanation was long and inadequate, you really need to play the games for yourself to get a feel for how much of a technological feat these games were for their time. Just trying to imagine the complexity that Bethesda faced during development makes my mind start to hurt.