In your personal opinion, what is the most underrated WOTC set. Whether it be overall value, underappreciated artwork, or scarcity that doesn’t show in the market. I want to hear everyone’s thoughts. I think Neo discovery is overlooked for overall value and has amazing artwork that should bring more desire to own the set as a whole. Discovery and Genesis sit in the shadow of Destiny and revelation and i think there’s alot of potential for those sets. Although it’s a subset I’d also like to point toward the southern islands set. It seems that the set is not well loved even though it has my favorite artwork hands down, yet the holos in psa 9 still go for well under $20 each (aside from mew). The pop on the PSA 10s for some of the holos is insanely low and new sealed sets are incredibly hard to come by so I really don’t think that pop report will move much at all in the future. My favorite set of cards for art and potential value in the future.
As a set, I’d say Base Unl easily.
I definitely agree. my only reservation with unlimited base is the sheer amount of product. Being able to pick up nm-mt holos for $5 each and psa 9 copies around $20 seems insane but the idea of 7-8 print runs makes me wonder if scarcity is even relevant in unlimited base. I think how iconic the set is drives its price. Artwork doesn’t appeal to me as much because i just don’t care as much for the backgrounds but man it’s iconic and holds some of my fondest memories from childhood.
Base set 2.
Fossil 
Base 2000/UK/4th
Even though its starting to get some love Expedition was looked over for a long time. I also love the look of the booster box and packs.
Neo discovery. Those art works. Frothing…
Legendary collection! The reverse holos are bad ass and I love the artwork on the booster box and packs. I just wish they would’ve used the base set blastoise instead of rockets dark blastoise. It looks out of place next to the base set venusaur and Charizard.
Gym Heroes
The Gym sets imo
Expedition. I feel like people don’t realize how hard it is to get that Charizard. 1 in 3 packs you get a holo… 32+ holos in the set. Combined with very low pop reports I’m supposed it’s not sought after.
Honestly when I think of WOTC, the word “underrated” doesn’t come to mind.
If I had to pick… probably Expedition or Neo Genesis
HGSS
Neo Discovery. Amazing artworks & pack art. Also Base 2.
I can think of at least two jump-out high-demand cards for each set except Gym Heroes, so that’s my pick. Erika’s Dragonair, Rocket’s Scyther, Sabrina’s Gengar, and two Moltres cards, what’s not to love?
The answer is Base 2, which is as low-rated as it deserves to be 
For me, Fossil and Jungle 1st editions - Artworks alone on jungle are amazing! So rich and colourful. Plus all cards are part of the original 150 and are all ‘rookie’ cards (couple exceptions of repeats from Base).
Currently prices for PSA 10 on eBay, in my eyes , are a steal right now!
on eBay, they hover around $15-$30 dollars each for nearly all common, uncommon, rares (few exceptions for non holo birds and eeveelutions - $30-$50). The Holos range $60 - $400max.
If you were to send a mint card (either from an eBay purchase or from your collection) to PSA yourself is practically 1st time lucky for a PSA 10 grade for literally half of each set! Pretty good considering the age of them now!
Also, the low count in the set (64 and 62 respectively) makes it very affordable to complete. They’re still low in pop count too (practically the same as 1st Ed. Base)
gym sets in my opinion
Neo era for sure. I say this in reference to when they came out and not present time. I was one of the very few in my school at the time with cards. Not even my friends cared for them or even knew the Pokémons at the time lol.
Base Set 2 for obvious reasons. They were the first set with reprints of previous sets, so a lot of people weren’t happy with them. I personally think they are underrated though. They may not be the original like Shadowless 1st edition, but they are still the same artworks, so I think we will see an increase in these if the Base Set cards keep raising.
Neo Discovery. Neo Genesis was the first Neo set, Neo Revelations has the Shining Magikarp and Gyarados, and Neo Destiny the Shining triple stars. Neo Discovery you don’t hear that much about though (in comparison), even though it has some great artworks as well.
Aquapolis (excluding the Crystals of course). Maybe it’s just me, but I hear a lot more about Expedition Base and Skyridge than Aquapolis…
Greetz,
Quuador