Which is the one Elder Scrolls Game that you would buy in front of all GAMES!?! WHat makes them so good?

For me? Morrowind.
Why? It's all about choice and consequences. That's what I really enjoy in an rpg.
And with modding, I can add further to that aspect of the game.

I adore Morrowind. Even when one overlooks the environments which are so often touted as "exotic", the amount of depth that went into the books, the architecture, the ecosystems easily dwarfs that of any other Elder Scrolls game. There are so many little touches such as carryable candles and hidden dungeon passages that really leave one to wonder what happened with Oblivion. Not in terms of gameplay, quest writing nor graphics - Bethesda improved and continues to improve in those areas through each installment in the series, but at the same time I feel as if these later games are stale in a way due to the amount of polish they have. For instance, dungeon loot in Oblivion is always stashed in levelled containers, and you will pretty much always find those containers in the exact same places.

Morrowind has so many little niceties in the design of its world and your gameplay choices - things like throwing darts and spears don't exactly make the game, but it's great to know that they're there. It adds both to variety in shops/NPC inventories but also shows you just how much the developers made it a point to keep the player amazed at the game's content, even through subsequent playthroughs.

Oh, but Daggerfall is the best of all these things if we're comparing the games unmodded. Modded, I'd go with Morrowind, but Daggerfall is my favorite vanilla TES game. And once DaggerXL is done...well...