Ultima VII is the very definition of deep. There's literally hundreds of hours of game in there, and beyond that a staggering number of pointless little things to waste time doing. It has a definite plot and an overarching quest, but there's no rush to complete it or move it along. It has the sensibilities of an online game almost, in its capacity for just being in the world and tinkering about.
There's a lot going on beneath the surface, too. The relationships between you and your companions, and they with each other. Your reputation and standing with the NPCs - Ultima VII was pioneering things that are still seen as revolutionary in contemporary games. I spoke earlier of Angband and its discarding of the extraneous content of RPGs. Ultima VII is where that content is exploited fully, and its welcome here. It's a game to truly lose yourself in, and so much fun can be had with it that it's always an enticing prospect.
This may all sound a bit heavy, but in fact Ultima VII is full of humour, in the conversations and interactions with other characters and the possibilities for mischief that the player freedom allows. Just thinking up new ways to be a nuisance can be fun. Alcohol is always a good source of entertainment, for starters.
When you're done with messing about though, Ultima VII provides a top-notch story and a massive quest that will take you far and wide. As an example of an epic, western-style RPG it's probably unbeatable.