Are you serious!? First of all, Haunter is indeed S-tier design, but so is Gengar???
Here’s a list of a FEW middle evos who have far better designs than their first/last evos: Wartortle, Dragonair, Arctibax, Dolliv, Rhydon, Dottler — I could go on….!
Am ready, willing, and able to fight to the death over these opinions, so come at me!!!