Something I really like is seeing the progression of the artstyle of manga artists.
While I like artstyle progression in general, I think manga is special in this regard because it is the same artist drawing usually the same characters for years, so you can compare how the design and quality changes over very long periods of time.
Below are a few different and distinct examples of how the artstyle of an artist or series has changed over time.
Artist: Hirohiko Araki
A great example of this is JoJos. The series has been running for over 30 years and it has changed drastically during that period, always staying with or even ahead of the times.
The JoJos throughout the years
You can see the main characters start buff and macho like an 80s action hero, then progressively get smaller. Distinct changes in shading as well, moving from a lot of very heavy shading to a small amount of lighter shading.
What I find especially interesting about this as well is that over the years I don't think Araki has become an especially better artist. If you compare the quality of say part 2 and part 7 the quality hasn't changed dramatically. Yes newer parts have more detail, but that I think is a product of both shifts in popular style, as well as new techniques and technologies.
Artist: Sui Ishida
This one is both a personal favorite series, and a fantastic one to compare because the artist actually redrew the entire first chapter years later so we can compare both the art itself, and the presentation of the series.
If we compare just two panels, the first appearance of the main character, it is obvious how much Ishida has grown in skill.
But not only that but the composition of the scenes is radically different as well. Characters are shown in different postures from different angles, and the framing is entirely different. The original version used only rectangular panels and were generally arranged in a normal grid tough they did vary in size and spacing.
The redraw however frequently breaks away from rectangular panels using various shapes and angles to frame the content in different ways.
These are of course but two examples of how artstyles can change over time. TVTropes has a whole page dedicated to the topic.
And of course I generally disregarded shifts in the overall artstyle of anime and manga. Kotaku (oddly) has a decent article with plenty of visual comparisons.