Going for another one here.. I'm going to use several parameters: Ingame obtainability, Ingame usability and Animé-Representation.
Drum-roll.. Mothim.
The male evolution of Burmy that just happened to
be.
Ingame obtainability:
- DPt: Get a Burmy from a Honey Tree to be male and your set at lvl20. Honey Trees though..
- HgSs: One special tree needs to be Headbutted on Route 38, and you have a chance of knocking out a Burmy.
- BW: Nope. Transfer, or Dream World.. Which has been shut down.
- B2W2: Nope.
- XY: Route 3, 10%. Not very rare, but you need to consider that in Santalune Forest, you can get a Caterpie, Weedle, Surskit and the new and improved thing that evolves into Vivillon.
- ORAS: Not available, unless traded.
So all in all, if you didn't want to be bothered with Honey Trees, you were unlikely to ever get a Burmy or Mothim in DPt. HGSS gave you the option of getting a Burmy if you knew where a very specific single tree was, so that's not giving it any praise either considering most would miss this. Generation V basically ignored its existance. Generation VI brought it back, but surrounded by all its Bug/Flying-competition: Caterpie, Weedle, Scatterbug (had to look it up) and technically Masquerain. All in all, this tells us that GameFreak doesn't really like this thing a lot, but threw it in - because they could.
Ingame usability: Vivillon's
right there.. And it's so much better, people tend to go for the new thing. The thing that Mothim uses is: Quiver Dance, Air Slash, Bug Buzz and Psychic. Vivillon and Masquerain can do pretty much the same, or better. Butterfree can do the same except Air Slash, but Butterfree gets CompoundEyes, meaning accurate Sleep Powder. It's as if GameFreak has denied Mothim its viability ingame as well.
Animé Representation:
Big surprise, this thing hasn't appeared outside of just
TEN episodes, all of which were during the DPt-era. Last time we saw it was on July 24th, five years ago.
That being said, I'll be laughing my butt off if anybody has seen it competitively since as well.
Looking into this, Stantler hasn't been seen since a few episodes before Mothim's big disappearing act: July 3rd of 2010 in the US.
I believe the 'Sense Fairies' from Pokemon X/Y are overlooked. Spritzee, Aromatisse, Swirlix, and Slurpuff are all very interesting. However, as pure Fairy-Types, they're overlooked for Sylveon, Florges and Xerneas. Xerneas stands as a buff sweeper, with Sylveon and Florges as special tanks.
So Aromatisse looks worse in comparison to both of those. Sylveon has many of the same toys as Aromatisse (Draining Kiss + Whole Team Status Cure in the form of Heal Bell/Aromatherapy) whilst also sharing a type. Aromatisse does have superior HP though, and is an epic Trick Room team member. I'm using this thing in Y, and it is a TANK. It can also buff with Calm Mind to become a deadly Special Tank that can take so much. Clefable also out does it, but again has inferior HP.
On the other hand, Slurpuff has its impressive movepool as a boon. I mean Surf and Energy Ball? Calm down, cotton candymon! Its lack of Moonblast severely hurts it though...
Then there's the fact both are trade evolutions, and require an item...
I don't really agree here. I agree with your analysis of Aromatisse, but it's not unloved. There's plenty of people who use it competitively, especially in doubles/triples/TR - and it's line isn't unloved in the animé either,
Spritzee being the first critter Ash sees when arriving in Kalos. Slurpuff is one of those critters that can actually pull off Belly Drum quite well, partnered with a Sitrus Berry healing it up quite a bit,
and the Unburden speed boost. Whereas Azumarill will remain Reliant on Aqua Jet to bypass foes. It also has access to Drain Punch, so set up the BD, you get 30HP back instantly, Unburden doubles your speed to acceptable levels, whack the opponent with Drain Punch and the 50%HP cost is fully replenished and you can spam the Fairy/Fighting-combo from there. And again, the animé hasn't shunned it either.
They're also not hard to obtain either outside of version exclusivity. Compare that to the analysis of Stantler I made.