I would put the City State of the Invincible Overlord first as the most solid and expansive out of all the Wilderlands Releases.
For the rest it gets complex and nuanced. First keep in mind that that the later the Judges Guild release is the more hands off Bob Bledsaw Senior and his original crew seems. There are definite tonal shifts so the entire line of Wilderlands stuff has inconsistent writing voices and unique flaws.
Wilderlands of High Fantasy
The first of the series and it shows. Not so much quality but in that they were figuring exactly what to do to present it. Map 1 CSIO is the most divergent and the format largely settled by Map 4 Tarantis and Map 5 Valon. Even the actual Map 1 was done way before Map 2 Barbarian Altanis. But luckily the format settled after Map 2. But the original Map 1 was shifted by a hex row compared to the later maps. Also WoHF is packed with a lot of supporting rules material.
Fantastic Wilderlands Beyond
The series hits it stride but loses it support material.
Wilderlands of the Magic Realms and Wilderlands of the Fantastic Reaches
Pretty much equal in quality, has the later Judges Guild layout that more polished. Feels like in some places they are just coming up with random stuff. For example there is an area with three villages of Lawful Good Orcs. Not quite close enough to explain as a some type of unique Orc kingdom so three separate explanations are needed.
Plus these two and FWB are what to get if you want to run same type of Argonaut/Odyssey style seafaring campaign.
City State of the World Emperor
I think this was done well, and until my release of Fantastic WIlderlands Beyonde, the only place where you get Map 6 CSWE. Map 6 has a minor flaw in that there were no castles or citadels except for two on the eastern edge carried over from Map 1. On the plus side we have a boat load of terrain description.
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Wraith Overload (CSIO Sewers)
Great idea, evocative content, mediocre execution. But salvageable with a little elbow grease.
The Wilderness Series
Mines of Custalon, Spies of Lightelf, etc. Uneven and mostly mediocre but the maps are pretty good and impressive in the level of detail they convey for 5 mile hexes. All of these are salvageable with a little elbow grease.
The Other City States
Modron is better than Tarantis and has bunch of underwater stuff as well. Tarantis is mediocre but fleshes out Map 4 Tarantis a bit.
Wrapping it up.
Outside of Map 1 and 6. My recommendation is to be prepared to supply the high level details about who rules what and what folks are doing. While for the most part the entries come across random they are also great inspiration. So use what inspire you. Interpret the others nearby in that light. Don't hesitate to change anything you feel is out of whack.
I used the Wilderlands across multiple campaigns and multiple system using what happened as part of the background for the next. The biggest change came a decade in (circa 1988) with my originals about to fall apart. I chose to redraw the maps by hand and expand the scale from 5 miles to 5 leagues (12.5 miles). With a league defined as an hour's walk. I did this after a comment by more than one group that the distance between CSIO and CSWE ought to be bigger the way I portrayed them in my campaign.
Later when the internet got rolling I met some who did that themselves and others who kept the 5 mile scale. But I think the Wilderlands works best when you stick with it long term. Doing that makes the setting your own in a way that doesn't happen with Greyhawk or Forgotten Realms as you come up with explanations for why different random elements exist the way they do.
