Red Dead Redemption defines an epic ending.
Yes. A thousand times yes. The ending to that game was perfect, from the final stand-off you thought you just MIGHT win (but was impossible), to the final scene on a river bank as a man fishes . . . Epic. The whole thing.
Red Dead Redemptions ending defined really upsetting, disappointing and unfulfilling. I was mortified that Marston was still gunned down after everything. He was also one of the best video game characters ever.
I take it you've never seen Butch Cassidy and the Sundance Kid? Heard of a Bolivian Army ending? Or read many westerns? Or looked at the history of actual cowboys and outlaws from the Old West? Happy endings are not a thing for the genre in general. Marston had been a criminal and done awful things. Sure, he tried to turn his life around, but it all caught up with him. You believed he had redeemed himself during the main quest, but for the genre "redemption equals death". Marston died for what he believed in, and he died along with the Old West. That may not stroke your ego as a player, but it is some of the finest story-telling put in a game so far.
As to the endings of Morrowind and Oblivion:
I never beat Morrowind's main quest. I played the game a dozen times, but I never got hooked on the main quest line. I doubt I even completed half the main quest.
Now, OBLIVION . . . well, Oblivion's entire main quest story was awful. Maybe not the exact plot, but the way it was framed and handled was horrible. For instance - your character in Oblivion did not matter AT ALL in the main quest. The entire plot of the main quest literally did not require your involvement in any way. Had you not been in that jail cell when the Emperor came through, nothing would have changed. Septim would have died in the sewer ambush, dropping his amulet. The surviving Blades would have taken it to Jauffre, the grandmaster of their order, to report the Emperor's death to their superior. Jauffre would have said, "Don't lose hope! A heir survives!" The Blades would have been dispatched to Kvatch to rescue Martin. They would have. Etc. Etc. Martin then does his big battle and self-sacrifice and the land is saved. Meanwhile, your character would continue to rot in the jail cell one over, unimportant to the world.
That is the DEFINITION of a bad game story, when you can remove the main character (the player) and nothing changes.
Will Skyrim's ending be epic? I hope so. As long a Bethesda never makes me a mere spectator again I shall be satisfied.