Oblivion has better mods, a better story, but the game play falls flat. Unless you get OOO and Simple needs. And they just broke so much stuff in Skyrim, like enchanting, and they flat out removed spell making in Skyrim, which makes being a mage over level 30 almost impossible in that game. Seriously though, oblivions mod community is 10 years old and still going, some really great mods.
I started replaying Oblivion a couple days ago, and I can say that the one thing Oblivion has on Skyrim is the quests. Oooh my god, these quests. After months of playing nothing but Kill Dragur, Acquire Currency simulator, playing the kind of quests Oblivion gives you is a breath of fresh air. They range from quests like grabbing rings of weight from the bottom of wells to sniffing out cults in the woods to finding the owner of a particular vintage of wine.
The atmosphere of the game is different-in-a-good-way too, and while the gameplay is kinda wonky (and don't get me started on the character's faces), I'd have to say Oblivion has more charm than Skyrim does.
The quest that makes everyone who has played oblivion smile is "Whodunit". If you haven't played it, I won't spoil it, because it is the greatest quest in the game. All of the dark brotherhood quests are. Oblivion was funnier, more varied, and kinda nutso compared to Skyrim and Morrowind. Those were serious games for serious people. Oblivion has a quest where you break into a dinner party and remove everyone's clothes with a spell. Those games ended a bit ambiguously, where as in oblivion, provided you did shivering isles (which is even more nuts than the base game), you became a GOD. No real room for death there. Also, in the arena in oblivion, a guy did the math, and OVER FOUR MILLION PEOPLE have to die for you to become grand champion, accounting for everyone else arena fights before you of course.
Yeah the Dark Brotherhoood was miles better in Oblivion then in Skyrim. If Oblivion had Skyrims engine and not the mechanics it'd probably win GOTY for me for the next five years.
So, fixed enchanting, seperate pants, and spell making? With stats and classes? I would never stop playing it. I already play oblivion at least ten hours a month, without fail. Got like 2349 hours into it by now, been playing it off and on for 7 years now.