What is the most unpopular/underrated Eeveelution?

The posters here have made a compelling case for a number of Eevee evolutions' unpopularity. I also think different groups of Pokémon fans are going to evaluate them differently. Designs might appeal to some, while competitive viability may make them appealing to others.

I think the original trio are always going to be popular. The same goes for the first two added in the second generation, even if Umbreon is more popular than Espeon.

I think Glaceon and Leafeon are the most likely candidates for least popular.

They came at an awkward period for the Pokémon series. It was in a lull after Pokémania had long since passed and a lot of the original fans had "grown out" of the series, at least for a time.

The games, Diamond and Pearl, weren't too popular compared to other entries, although the DS's success and appeal outside of a traditional gaming audience did give them a bump. For many here today I suspect they may have been your first games, so maybe they are the classics for you.

In game, their evolution method was novel but a little awkward, requiring the player to go to a specific spot in the game world. Some may not even have realised that Eevee had new evolutions at the time. You are unlikely to evolve an eevee in that exact spot by accident.

Finally I think their designs are a little obvious and understated. I like them, save for Leafeon's eyes which I find really creepy, but they are a little obvious for an Eevee design. They could easily have been fan creations.

Sylveon however came during a bit of a resurgence for Pokémon. The 3DS was beginning to gain traction after its initial commercial failure, it was the first time a main series Pokémon game had appeared in full 3D (even if the game world's design was still rooted in 2D principles) and it introduced novel evolution methods on the new hardware that specifically required intimate interactions between the player and Pokémon, namely Pokémon Amie. That would have helped cement Sylveon, and the player's own specific Eevee they interacted with to turn it into Sylveon, in the player's mind.

In short, I think either Glaceon or Leafeon are the least popular Eevee evolutions, because they represent an awkward middle child in the Pokémon series as a whole and the Eevee line.

Thank you for coming to my Poké-TED talk.

@JohtonianGuy Oh, yeah, Sylveon was the poster boy for the new Fairy type, wasn't it?

I keep forgetting that it's Fairy type, not because I forget Sylveon, but because even after a decade, I still keep forgetting Fairy type even exists and that numerous legacy Pokémon had a type change.

I'll hit a Clefable with a fighting move and suffer a moment of confusion when it's not very effective, only to realise... "oh yeah... Fairy type."

But yes, people who like Fairy types also tend to like Sylveon for that reason.