Well... I'm a bit biased. I have played Oblivion a total of four hours. It just didn't and hasn't grabbed me. Boring environments, boring plot, boring mechanics. I played Skyrim religiously for some 150 hours, and loved it to death for about six months. That was five months ago, and I've never been back, nor felt the need. But Morrowind... I played it first, bought it two years ago, finished the main plot and nearly every guild quest eight or nine times... and I just played it again today, and will again tomorrow. I love it that much. I count it as my personal favorite game of all time.
I will fully admit that some aspects have aged poorly; a few wonky or outright broken mechanics are present, some rampant balance issues, and engine glitches are usually hallmarks of the older RPG genre, and they're all present and accounted for here. But Morrowind is the rare game that is more than the sum of its parts.
Morrowind has a richness and a depth to it that I just haven't been able to find anywhere else. Many people malign the game for its lack of fully voiced dialogue and its large amount of reading, but I actually hold this to be one of its greatest strengths. This allows for it to show you an incredibly deep and interesting story to its lore, and its main quest could easily hold up as a series of novels in its own right. All of which set in a unique and interesting culture that, unlike Skyrim and Cyrodiil, is not an easy equivalent to a real-world culture in history. And it's all set on a huge volcanic island where Dunmer live in ancient crabshells and build giant mushroom cities. It has an utterly unique atmosphere that you're just not going to get anywhere else. And unlike Oblivion, the world is actually static and its places interesting, making it worth exploring. Especially without any glowing arrows and compasses nagging you to go a certain way; the only way to know what's over the next hill it to go look for yourself. I simply cannot overstate this enough; Morrowind is unlike any place you've ever explored before.
Another aspect usually talked about with disdain is the combat. While I can see the reasoning, I also very much enjoy it for reasons that most people forget; it's an RPG. This is a sad side-effect of the game moving to a first-person perspective and not making use of a party. Nobody maligns Baldur's Gate or even Dragon Age for not being able to feel the shock of impact traveling up your arm as you execute an acrobatic flip and bury your blade into an enemy skull. If that's your thing, I can recommend a couple good hack-and-slash games. But this is an RPG, and that means numbers, dice-rolls, armor-class, skillpoints, hit chance, damage tables, weapon degradation, and strategy. All things that many gamers forget. Yes, balance issues exist, and yes, it is not as involved as some. But the combat in Morrowind was a lot more successful at making me feel like a warrior than Skyrim ever did.
And finally, graphics. Many people malign Morrowind's graphics, but this one I don't understand. Certainly they are not photo-realistic by the ridiculous standards of today, but Morrowind is the best kind of old games; the graphics may be dated, but they look beautiful in their own right. For me, Skyrim looks like everything from the trees to the character's hair follicles are hewn out of stone; cold, hard, and static. For me, everything in Morrowind is crafted out of folded paper; warm, soft, skillfully made, and beautiful. And as I said before, the atmosphere is unlike any other fantasy world I can think of. It's a place with its own history, and its own identity. Playing the game feels like coming home.
A word on mods. To be honest, I probably wouldn't say half these things without the game's modding community, but there it is. This game has one of the oldest, most well-established, most dedicated modding communities for any game in history. The game is very easy to mod, and there are thousands of mods that either fix or enhance the game. And more are being made to this very day, as a testament to how much the game is loved even now. There are several even add entire continents, something you'll see maybe once or twice for Oblivion and Skyrim. The Morrowind modding community builds on what is already an excellent foundation and builds things that must be seen to be believed.
Seriously, I could go on for hours on the superlative awesomeness of this game. I recognize that the appeal is not for everyone, however. But I heartily recommend you give it a chance.