IMHO, the best feature of Oblivion is its sandbox nature. It is ideal for deep roleplaying where goals are secondary and the concept of 'winning' the game has no meaning. It's about living day to day and searching for purpose and fulfilment.

In Oblivion, I see myself as a guide for the interesting character who showed up in prison. I have to actually like that character in order to play them. I could never play an assassin because I detest psychopaths.

With most games there is the fantasy element of magic and I love being able to do things I could not do IRL. Oblivion delivers those larger-than-life elements nicely packaged in graphics and sound effects. For me, it just never gets old.

I personally came back to Oblivion recently because last time I modded it in 2009/10 it was a mess, so I gave up, moved on and eventually got Skyrim and modded it.

Now, it seems that it may be easier to get a stable and massively improved Oblivion game since there are a lot of tried and verified stability mods/ tweaks. It still however, took almost 2 months for me to get Oblivion working.

I quit modding Skyrim because I feel the modding scene is not mature enough (even with S.T.E.P) to allow a stable and heavily modded setup on my rig. I can wait another 2 years

TLDR; Came back to Oblivion due to modding stability.