Every GTA except for Vice City (and San Andreas),
Dark Souls (the whole shitty franchise),
Fallout 3 and 4

Edit: Heh, I almost forgot about the biggest disappointment of them all, God of War (2018), or rather Dad of Boy (2018)...what a huge, steaming pile of shit...

The most recent version of Sim City (2013).

One of the most recent ones was replaying Knights of the Old Republic with a dark side character. As most people probably do, I had good memories of my first playthrough with a character on the light side of the Force, and I remembered liking the story, even though the big reveal was not really that surprising at the time I played it. Anyway, playing with a dark side character makes it painfully obvious how the player has absolutely no impact on the story (except right before the end, but it doesn't really change all that much), and not even does it feel like you are just a puppet on strings, the game even freely admits this! Another thing is, no matter how evil the things you do are (and much of the "evil" choices are just things like steal from poor people or steal from them and then kill them), the worst that can happen to you is that your Jedi companions give you a lecture. Also, the controls are consoly and the inventory is horrible, and there is not really any depth to combat or the character system, despite their D&D origins.

Can I just press you a little about Undertale? As to why you found it a letdown?

Lol this guy is trying to be edgy. What were you expecting to be disappointed?

I was expecting a good story as this was supposed to be the best game of all times for some.

In the end, I appreciated the awesome soundtrack and some really cool characters, but all the metagame about how monsters are nice, to be honest, was very childich for me. I mean, I have empathy for monsters since pretty much the first game I played. I remember wondering why exactly we had to kill this monsters in the first quest of Diablo II and if they had a family or so. It remembered me my childhood when I was 7 or 8 and I was "designing" in my head such games / worlds where monsters were nice. So basically when it came the moment of the big fat revelation at the end, I was like "yeah OK so it was a story for the 7-year-old me" which was quite a big disappointment. tI was like a big deja-vu.

If you add to the top the friendly battles which are nice for the first hours then become just boring. Then the "tricks" and puzzles of Sans that we have all already seen in different games or books or movies (stone that does not want to move, etc.)... it was highly frustrating to just discover absolutely nothing really new to my brain.

I'm pretty sure I would have more appreciated the game if it was a tiny indie game (as it was intended to be by the dev, I assume). But too many journalists have praised and over-intellectualised the game for me not to expect something more than the question I was asking to myself when I was a child. In the end, I felt like the game was pretencious, and I'm quite sure it's only because of the awesomeness I was expecting - not because of the developers' intentions. On the contrary, for example, The Witness has been told pretencious and was in the end one of the best videogame I have ever played... Va savoir...

Hope I have answer to your question. I really tried to capture the essence of my deception
(and translate it into English so I hope it can still be understood...)

Grim Dawn. A highly praised ARPG said to offer the finest gameplay mechanics in the genre. Apart from nice visuals and diverse character builds, I wasn't impressed in the slightest. I played an Occultist/Druid and just obliterated everything with my fully upgraded Dreeg's Eye, with my summons mopping up everything that didn't explode. Wasn't even a challenge on the normal difficulty. The quests were boring, of the fetch variety, and the maps felt like a slog. I didn't care much for loads of dropped loot as 90% of it was just inventory clutter. I didn't care for the same thing either in Borderlands, but at least that game was funny so that kept me going.

I guess ARPGs just aren't for me, even though I did enjoy Nox very much. I guess the lack of excessive loot is key here.

Ah, so you don't like it for actual fair reasons. Fair enough.

Of late? I'd say Alien Nations due to its difficulty. That being said, I went into it with low expectations after the demo gave me some fits in the early 00's.

Somehow forgot about Commandos 3. Easily the game that disappointed me the most. Commandos 2 had huge intricate levels that you could enjoy in 1280x1024, so I was completely confused when I wasn't able to increase the resolution in C3 to more than 800x600. But I soon found out why: the levels are very small compared to C2, at least all of the levels except for the Berlin level. You also use far less of your team on average per level compared to C2, and one of the most popular commandos, the marine, is only available in one short level. Exploration was toned down as well, with way too many doors that couldn't be opened. I think I only found one door in all of C2 that couldn't be opened to explore what lies on the other side. Not that C3 is a bad game on its own however, it just pales compared to what preceded it.

If I ever actually played Commandos: Strike Force, then I imagine that would count as the biggest disappointed, but as I didn't I'll go ahead to pretend that it never happened...

GTA IV

Mount and Blade. I read and heard so many great things about it that it drove me to purchase it at one point and could not for the life of me figure out what others were seeing that was great... ugh

On and Mass Effect 2 as others have said.

I think it must be GTA 4 for me as well.

Quite honestly, anything that was released in the last five years or so, perhaps with the sole exception of Witcher 3. It just ticked all the boxes of what I enjoy, worked properly throughout and has been great replay value.

I probably would have enjoyed GTA5 much more, were it not for the utter mess called Social Club. I wouldn't mind playing it again, but I refuse to install that awful DRM abortion on my system ever again. Most of my issues revolved around getting to play SP properly, as I'm not the slightest interested in MP.

Kingdom Come: Deliverance had me excited at first, until I ran into the plethora bugs and problems. Typical rushed out the door release that happens a lot these days, plus I just didn't like the combat system. The concept of it was appealing, but the execution was abysmal and sucked the fun out of playing.

Either I'm getting burned out with gaming and lacking patience as I get older, or most of what's coming out these days is simply garbage. Very few releases in the last few years appeal to me, and those that do have usually been trashed in reviews that I've read. Mostly due to being riddled with so many problems. That means I don't pre-order or buy anything early these days.