Here is a similar thread from a year ago, which mentioned some nice underrated cards.
I personally think the SH shining Pokémon from the Stormfront, Platinum, Supreme Victors, and Arceus sets are pretty uderrated. Some examples:
And of course the matching Altaria for the Swablu:
Milotic as a Pokémon in general is pretty underrated as well. Personally I think it’s the most majestic Pokémon there is. Here my three favorite artworks:
And this Arceus set I bought last year is pretty cool as well, especially with the pattern going over all cards:
But I can probably name thousands of underrated cards. 
Greetz,
Quuador