Castform because as I have said before it is hugely diverse, doesn't have terrible stats and sunny castform can deal X5 damage with weatherball.
Walrein - It can take a beating, has good typing, good stats and has a pretty decent movepool.
I have used shifty and walrein a number of times and they are actually really awesome. Quite a few of my friends actually use these pokémon a lot too so I wouldn't call them underrated. I would have to say in some people's minds Crobat is pretty underrated and has absolutely amazing speed. Also Exploud, even though it is a normal type, it has decent defense and attack stats and can learn some really good moves.