I would have to go with the first Final Fantasy, Xenogears, and both Knights of the Old Republic games. Ironically two of the four (Xenogears and KotOR 2) were rushed and could have been even better.
The original Final Fantasy got me into playing RPG video games back when I was into just the tabletop ones. That's part of the reason I started playing RPG video games like I did. Games like Final Fantasy 6 and 7, The Secret of Mana, the Lufia games, and Chrono Trigger might have been better and even had better plots, graphics, and gameplay, but they probably didn't have the impact that the original did on my personally. It was also the first game I spent countless hours playing. It was also one of the first games where you had to put more time into winning and couldn't just rent it and win it in a day or weekend like 99% of the other games seemed to be like back then.
Xenogears had one of the best plots in a game ever. What's funny is that on it's surface it seemed like it was going to be a game about fighting in giant robots, and shortly into it you could already tell that was only a very small secondary part of what made that game what it was,...and yet they interjected in such a great way that it keep the giant gears in it cool at the same time, and in a way that totally made sense. Xenosaga while it had potential just never lived up to it as a spiritual successor and it's a virtual crime that both Squaresoft and Namco didn't give Tetsuya Takahashi and his wife the freedom to create the series like he wanted and they jerked him and the game around to ruin the story and to maximize profit. If either of those studios had more than a couple of brain cells they would have put the resources into it and let him create it in the matter he wanted. It would be one of the if not the greatest masterpiece that a video game ever produced, especially story wise.
Knights of the Old Republic and it's sequel pretty much saved Star Wars for me at that point. I loved the Star Wars Extended Universe, but at that point they were screwing around with it with the start of the New Jedi Order series of books and making a mockery of the once great thing it was, and they interjecting one dumb ass thing, idea, or concept after another, ruining characters or stupidly killing off the worst ones to do so left and right, and I was about ready to just wash my hands of the whole thing. Then came KotOR and not only did if feel like Star Wars and give something about it to like again it ended up having my second favorite fictional character ever in Revan. In fact with very few exceptions the whole cast of characters I liked. Then came the comics from it which most of the most dedicated fans things are better than the books now anyhow. Of course where getting back to where they were and are going to ruin that era of Star Wars with the money grab that is the new MMO coming out, but at least for a little while it made it good again. For keeping something I loved that much from totally being destroyed it deserves it. I don't know what it is with the authors of the novels and the powers that be in control of the Star Wars EU, but they just can't help themselves when it comes to screwing up something that's great about it just for the fact they want to change something or maximize profit by catering to the most casual of fans that don't give a crap one way or another. Not having a decent and quality continuation of the story from those games preferably written by Avellone and/or Drew Karpyshyn is another virtual crime.
Honorable mentions go to Neverwinter Nights, Neverwinter Nights 2 (I went into Neverwinter Nights 2 thinking I wasn't going to like it and was pleasantly surprised.) and their sequels and expansions, (even if they had flaws and were buggy) the original Legend of Zelda, the original Castlevania, Fallout, the original Super Mario Bros. which came with the regular Nintendo I had way back in the day so got played a ridiculous amount. I can still hear it's background music in my head when I think about it after all these years.
I have yet to have the pleasure of playing morrowind, but Oblivion has definitely been my all time favourite game since i first got it for the ps3. There's just so much you can do and so many different paths to level up on. It completely wrapped me up
To be completely honest I thought Oblivion was the most overrated game I have ever played. It might be the most overrated game ever. I couldn't find many redeeming qualities about it. :2 cents: