I’ve always loved Marill and Ivysaur. IMO, the design for Ivysaur is 10x better than Venusaur. And let me just add a middle-stage evolution I hate: Combusken. Good lord, Combusken is ugly.
Oh my goodness idk how I forgot Rhydon. Rhyperior is HIDEOUS. I don’t know how such an iconic Pokémon evolves into this horrifying hamburger patty abomination. Sinnoh is alright for the most part but they did our boy Rhydon completely wrong.
My favorite and in my opinion, the greatest designed middle evolution in pokemon would have to be Braxien from Generation 6. Delphox on the other hand was a bit poorly executed with the mage aspect compared to Braxien… I may be a little biased since it’s one of my favorite pokemon in general though; I believe a lot of people love Braxien over there in Japan as well!
Personal Favorite Braxien Card (XY Breakthrough, Kawayoo)
This is always kind of a weird topic for me because most of my favorite “middle evolutions” are the ones that only became middle evolutions retroactively, like Chansey. To me there’s a pretty clear difference between something that was intentionally designed to be a middle evolution like Ivysaur and something that accidentally became one years after the fact like Electabuzz or Magmar.
Of the first group: Bayleef, Wartortle, Braixen
Of the second group: Chansey, Jigglypuff, Roselia
Togetic is kind of a gray area to me. It’s one of my favorite middle evolutions and while yes at one point it was a final evolution, it always felt like more was going to be done with the line. Togetic’s design and even its name (Japanese name Togechick i.e., not a fully grown bird) always had me waiting for an evolution, and then they gave us Togekiss.