Ultima 4: Quest of the Avatar

This was the first Ultima game to feature a real conversation system--whereas NPCs in the earlier parts would only give one canned answer when talked to, now players could interact with them by specifying a subject of conversation, the subject determined either by a standard set of questions (name, job, health) or by information gleaned from the previous answers, or from other characters. Many sub-quests were arranged around this.

The game is also notable for setting the world and tone for the games that followed in the series. After Ultima IV,
the storylines became more and more linked, with many details from
earlier games referred to in newer games, often in a self-explanatory
way so that the player doesn't necessarily need to understand
everything in the previous games, even though it helps.

The world of Britannia was first introduced here in full, and the world
map in the series did not greatly change any more from this point
onward.