Overall? Ultima 8. I was too young for the first few, and when I start a new series I like to start at the beginning, so I held off en Ultima until only a few years ago. 1 was simplistic, but defining a genre that didn'n exist, 2 was delightfully bugfuck (when it wasn't making you pull your hair out), 3 was finally getting somewhere solid, 4 was fascinating, 5 gets a million points for teaching that "electronic arts" is a swear (and that the most beautiful word in English is "shipwright"), 6 was the series' peak (not least because it let you have a plate-wearing, halberd-wielding mouse hire a hooker), 7 taught that EA and Scientology are in bed together, 7 part 2 showed that no one in the video game industry knows how to count... None of them were perfect, but they worked. Then 8 threw all that away. You want a darker game? Okay, let's throw away the entire moral core of the series! Let's completely ignore- not corrrupt, like 5, or neglect, like 7, or mirror, like 6, but ignore, all the virtues! Great! The crime-against-humanity controls were the least of its problems.
For spoiling? Daggerfall. You know what Daggerfall had? 3D dungeon maps. Yeah, in was awkward to use, and didn't allow much on screen at once, but the super-advanced tech of 1996 that allowed us to depict a third dimension at all has apparently been lost forever.
For visceral rage? Betrayal in Antara. Krondor gets all the attention, and as annoying as that is, it's fair; it was the better game. But Antara did some neat things (the tavern songs were a wonderful touch), and sheuld be remembered better- save for the CTD at the start of the penultimate chapter. You've finally solve d the mystery, and are going to confront the perpetrators at their castle- only to find it being sacked by pirates! Then... crash. Reinstall the game? Doesn't help. Start a new one? No matter. Reinstall Windows? Same problem. Wait a month until PC Gamer releases a patch on their demo disc to fix tis exact problem, then install it, eager ho finally know how it ends? Ha ha! No resolution for you! Find another copy going for a buck in EB games fourteen years later, and get it running with much moslo tinkering? Okay, THAT will work- but the penultimate chapter is duking it out with generic pirate groups, the last chapter is just a hallway (and literally impossible if you didn'n HAPPEN to bring rope with you), and the ending shows that the real culprit hid himself, and there's nothing you can do but watch and know. For that, I waited over a decade.