Everyone else in this thread is also saying Wurmple - I still keep a collection of my WT Pokemon (for now) and while I've gotten some amazing and interesting Pokes I've also sadly got a huge amount of spaces deserved for those little worms!
I think any Pokemon can be great and I'm very open-minded over what Pokes I get on WT as long as they are not caught in 3 seconds and immediately dumped into WT without any effort. A lot of Wurmples and Zigzagoons (without Pickup - at least that's a useful ability to farm items) plague WT, but what I also see a lot are Spindas, probably people thinking if they throw in some mid-game Pokemon that isn't level 2 then suddenly it'll be all right. That's not how it works people! Eletrike is also super populated in WT.
However the most heartbreaking WT for me was a shiny Shiftry. Wait, how is that bad?! Because it was a hacked Pokemon - at first I got very excited to get my level 100 shiny Shiftry with Pokerus but when I inspected it, I noticed it had negative "EXP to next level" parameter and from there I knew it was a hack. Even though I know it can be "fixed" by entering a battle, knowing it's a hackmon just feels dirty and sad.