Basically this includes Grimer/Muk, Gulpin/Swalot and Trubbish/Garbodor ^^ Personally my favourites and Grimer & Muk because Grimer is such an awesome Pokemon in almost every way, and then Muk makes up for Grimer's weaknesses by being kick-ass good in battle! Garbodor is also quite a strong contender for me, but its stats and moves just aren't good enough. D:
Now, I would love to be a little original here and choose something like Garbodor, but I honestly don't like it one single bit. Imo the design shows that Gamefreak is running out of ideas, because a Pokémon based on trash isn't exactly the greatest idea they've had up till now.
I'm more neutral towards Gulpin and Swalot, as I find their design ok. It's not truly horrific like some other Pokémon, but it's not super good either, but I never really got to know them while playing the games, so I don't really have any opinions on them, and I don't know how good they actually are in battle. I find their concept pretty original though, so that's one of the probs they get from me.
Moving on to my actual favourites of the bunch, and the reason why I'm NOT being original, is Grimer and Muk. I've already pretty much made it clear in several of my previous posts, that I am indeed a lover of the original 151, and that is also one of the main reasons that I like Grimer and Muk. The reason that I like them is basically just, that they're the ones I've know for the longest time and the ones that I know the most about. Their desgins were pretty unique back in the days, though they're not really anything special when compared to all the other new Pokémon, but I still love them just as much.