GameSkinny JTP veteran and Senior Mentor Greyson Ditzler had a tough time picking his favorite Water-type Pokemon for this list. And if you take a look at his choices, you'll see that he managed to sneak a second Water Pokemon into his choice for the best Ground Pokemon.
Interestingly, Greyson chose the half-evolved Graveler as his favorite Rock type, rather than the fully evolved Golem. Perhaps this is because in earlier Pokemon games, when Geodude and his evolved form were still pretty powerful, there was no way to get a Golem without trading it to another person to trigger its evolution.
Other than that, this Pokemon fan seems to have a wide range of Pokemon and generations across his favorite types.