I like Skunktank alot , though everyone has their opinion. Personally I think it's underrated and it's not bad of a Pokemon. That's just me though.
Anyway, it's hard for me to just pick one, there's a few I really want to get rid of. Like Burmy and Purugly. But if I have to pick just one, I would get rid of Heatran. It's pretty much pointless and doesn't fit with the Pokedex to me. It has no significance to the plot of DDPt and seems like one of those Pokemon that Gamefreak gave Legendary status just to add another Legendary. And I only catch it just cause it is a Legendary Pokemon and is rare.