Here all the middle evolutions I like more than their first or final evolutions for three-stage evolution lines:
- Wartortle
- Pidgeotto
- Pikachu
- Clefairy
- Jigglypuff
- Haunter
- Chansey
- Mr. Mime
- Omanyte[1]
- Dragonair
- Quilava
- Marill
- Grovyle
- Loudred
- Lileep[2]
- Anorith[3]
- Dewott
- Krokorok
- Tirtouga[4]
- Gothorita
- Bisharp
- Floette
- Steenee
- Raboot
- Hattrem
- Floragato
- Tinkatuff
With most of these, I like the middle evolution just slightly more than one of the other two stages. 
The only ones for which I like the middle stage way more than the other two, most of which have already been named, are:
Haunter (my third favorite Pokémon in general). I’d personally rank the evo line as: Haunter > Gastly > Gengar, although all three are very good of course.
Hattrem. Amazing design, with her walking by using her hat/hair.
Loudred. Not too big of a fan of this evo line in general (it’s okay), but Loudred is way above Whismur and Exploud in my ranking.
Greetz,
Quuador
