I believe Oblivion is the best ES title

I played Morrowind and Skyrim. Morrowind was a pure nostalgic and retro experience that I liked a lot. It was an amazing experience to follow up the story only myself and not with marks, obvious hints etc.

Skyrim is not a bad game, at least I wouldn't call it that way. But it's just "follow the marks and you'll be ok". Oblivion is just in the middle of the two in this manner. You can still proceed following the marks, but it's not as easy as in Skyrim. Marks does not always directly point on the target.

The side quests are much detailed in Oblivion compared to Skyrim. Lots of dialogues, voice lines, all well designed. The thieves guild questline was amazing itself. Seeing the micro effects of my actions is the most fun part.

I was expecting the dark brotherhood questline to be generic, but no. It's fully voice-lined, connected to the open world. Every assasination is unique on itself.

The only cons I have seen in the game is that it's character mechanics are not as detailed/fun as Skyrim's. Skyrim's skill system was fun and great, meanwhile it's just(or feels like) pure stat increments in Oblivion. Skyrim's alchemy, crafting and such mechanics were basic and nice.

Oblivion is great in the manners of story telling, exploration and such open world concepts. But it lacks well done mechanics, maybe due to technological limits.

As conclusion, I believe Oblivion is the best ES title. Because, it offers a well-done open world with lots of stories to be told to the player. It is not as heavy as Morrowind is, or as light as Skyrim is. It's just in the equilibrium of an ideal open world RPG game where we play a hero.