Ultima V: Warriors Of Destiny. I think it was actually made more atmospheric by the simplistic presentation, because there was nothing between you and the plot, and the scripted characters you found in Britannia. And there were so many little delightful vignettes you came across as you traveled... I can still recall traveling back out of the Stygian Abyss to return a party member, just so I could come back and bring Captain Johne from his shipwrecked vessel, back into the world and beyond his mortal sins...
I loved all the Ultima series after IV to be honest, but I played IV on the Sega Master System first, then Ultima V on the Amiga, and VI on the PC, so it's V that gets the Amiga accolade for me. I actually bought Ultima VI for the Amiga second hand from a store in Stoke, around 1998, when I was still using it as a games machine. The game didn't work though, so I took it back, and the spotty oik behind the counter must have thought I was trying to copy it, and said I could only have a copy of the same game. I said that was fine, I really wanted Ultima VI, and he asked me to come back the next day. So I did, he gave me "another" copy, which also didn't work and was I suspect the same copy, so back I went for a refund, and remained disappointed until around 2002, when I finally got it on the PC... I think I ended up with SSI's "Sword Of Aragon" instead from that shop, which was colossally crap, but I remember making it into something fun over the next year regardless... Probably because my final year at Uni was just so unhappy, but that's another story.