I think Magmar, Haunter, and Graveler look somewhat cooler than their final forms, and the others for the most part are cute while their final forms are remarkably hideous/goofy.
I know everyone out there likes at least one prevo more than their evo.
dragonair looks awesome and can use the weather to it's advantage. Remoraid is a friggen cannon and is the most extremes number 3 in the top ten parasites. Tyrouge is given props because I always remember the time I unknowingly bred it with high jump kick. Charmander has been my favorite pokemon since I was 4.
Togetic, due to the fact that I raised one in Gold and one in Fire Red.
Oddly enough, I thought for a while that Togepi learned Extremespeed, because I didn't know that my Togepi used it through Metronome. Imagine my suprise when I found out Togekiss could learn it!
This is actually a pretty sad answer I'm forced to give. The competitive aspect of the game has been burned into my brain so hard the only honest answer I can give is Trapinch > Vibrava.
Probopass, Lickilicky, Tangrowth, and Honchkrow are basically just the gross-looking inflated versions of their pre-evo's..the pre-evo's all look so much better.
Meowth: I actually never evolved my Meowth in PMD1 because I like it so much more than Persian.
Koffing: Koffing is a jovial ball of poison gas, and seeing it smile makes me happy. Weezing is just some hideous, depressing mass of toxins and genetic mutations that I can barely believe once was my happy little Koffing.