Alright, difficult one. Let me try to explain...
My favourite Ultima is Ultima VI - The False Prophet.
Why? Because...
1. the technological leap from U5 to U6 was insane at the time. The game world was just so detailed and beautiful, and the music!
2. I loved the story. I still think it's the best of all Ultimas (yes, I know I'm probably in for a beating now
). Not your typical hero-against-the-ultimate-evil, but focus rather on fighting against prejudice and racism (including your own). Up to that point, it was always a fact that demon-like creatures had to be the bad guys.
3. The karma system: you could get stuck at the end of the game for not acting like you should have as the Avatar. That was harsh punishment, but just right IMHO (yes, I learned that the hard way back then, good thing there was SSSH to help me out
).
4. It was released at a certain time in my life, when I wasn't a little kid anymore, started to grow up, and for maybe the first time, was fully able to understand (1) the more subtle nuances of storytelling in a video game, and (2) the English language
.
Now, that said, while U6 is my
favourite Ultima, I think the
best Ultima is actually Ultima VII - Serpent Isle.
Why? Because...
1. Graphics and sound were again way better than in U6, and nicely improved over U7 BG as well
2. The sheer scale of the worldbuilding and all the detail was mindboggling
3. Npc chatter was much more polished, and the interaction with the world was overall improved
4. I found the task of finding your friend Gwenno much more appealing than hunting down Electronic & Arts. I still prefer that personal touch in games nowadays, like in Witcher 3 (Geralt looking for his daughter, not Geralt being the Chosen One to rid the world of an Evil Overlord).
5. Speaking of personal relations: Dupre's sacrifice was one of the most intense moments in gaming history for me. I mean, I had known the old man for years, and he had always been my favourite companion out of the three, so this really saddened me.
6. It had its flaws (combat & inventory management), but all in all, the game mechanics worked great for an isometric rpg, and still hold up today. Plus it had some quality of life improvements, like the better paperdoll inventory, or bigger and more detailed character portrais on dialogue, for example.
7. The Silver Seed addon was really cool.