Most people tend to like the third part the most, but to me, "Discovery" is even more atmospheric and has many more "whoa" places and moments.

The structure of the city underwent a major revamp in comparison to the first part. It's much more pleasant to move around and no longer as cramped. The crowds are still there, but they were made neutral to Garrett to make it less annoying. In addition to a series of impressing alleys, courts and nooks, there are several locations which really stand out, the tavern being the cream of the crop. It still looks and feels amazing and full of life even today.
The mission also has several nice, little, playable rooftop sections, and it's a decade before such settings become trendy.
There really is a number of things about "Discovery" which are definitely ahead of their time in FMs.

There aren't any trouble with sound propagation anymore and background music is among the most memorable in FMs.

On the flip side, as in the entire series, the bulk of your challenge will be in terribly unexciting secret hunt. While the story delivers, and personally I've found "Discovery" somewhat more rewarding than the previous mission, it's still definitely not the kind of a challenge you're looking for in an FM.

It's the point where the series really picks up, though, so it's absolutely recommended to play, even if in need of a walkthough to finish it off.

Try Keeper of Infinity 1: Bad Venture (now under Keeper of Infinity 2: Missions 1 and 2 - Raven's Nest (1st mission)) for the best in-game pub.