Charmeleon: I'm not sure why, I've just always liked Charmeleon. While it might seem plain, I guess I just like the name and think it looks pretty nice.
Haunter: I didn't have access to trading with people until Black & White, so I never really had much access to the trade evolutions back in the early days unless I traded with myself on the Pok?mon Stadium games. As such, I got used to using the middle evolutions, and Haunter's great. I love the design and name and, while I still like Gengar, I'd say Haunter remains my favorite Pok?mon of its evolutionary line.
Dusclops: Dusknoir is cool and I'll use it, but I prefer Dusclops aesthetically. I like the one cyclopian eye and the sort of mummy design Dusclops has. Nice Eviolite user as well.
Vigoroth: I like Slaking, but sometimes you just don't want to deal with Truant. A year or so ago I made a team and was trying to decide whether to use Vigoroth or Slaking and decided on just using both for their own different purposes. It's a nice contrast to Slakoth and Slaking.
Doublade: Say what one will about the design just being two Honedge slapped together, I like the stats, especially with Eviolite.
Dartrix: That hair is amazing. The Rowlet line is one of my favorite starter lines alongside Squirtle's (because of Blastoise), and birds are my favorite types of animals, so I just really like it.
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Other standout middle evolutions include:
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Electabuzz: I'm one of those people who is fine with Electivire, but Electabuzz is definitely my favorite of its evolutionary line. The design is memorable and it honestly would have been fine without an evolution, but it got one so here we are.
Magmar: Same as Electabuzz.
Dragonair: I like Dragonite, but it feels like too drastic a departure from the designs of Dratini and Dragonair and would probably be better as its own standalone Pok?mon, or the final evolution of a different evolutionary line. Dragonair looks elegant and it's not that bad with Eviolite.
Dewott: Definitely the standout of the Oshawott evolutionary line for me. I don't have much to say about it, the only starter I really like from Generation V is the Snivy line because of Serperior, so yeah. I just prefer Dewott's design over Samurott's.
Herdier: Just a good, simple dog.