I have quite a few faves of the Ground-type.
Sandshrew/Sandslash - I love how cute these things are and the fact they still remain a fave after all those generations, and I even used one to help defeat Elesa in my run of Black 2.
Diglett/Dugtrio - Strong, fast and really odd. Three things that fit in one description of this mystery Pokemon that has never had its full body revealed. Still a fave of mine today.
Gligar/Gliscor - I loved how Gliscor gained a useful ability in Poison Heal, now it sees use as a great physical wall with some neat reliable recovery tactic in the form of a Toxic Orb. Plus it has the power to dish out damage too!
And all of the 5th Gen Ground-types in the list as well. Krookodile and Excadrill are powerful, fast and have great abilities, and Stunfisk is just plain hilarious but I love that cutie. This generation made me love Ground-types again I have to say.