I see I got a special request to answer this! :P
Now, the basic idea of any Ground-type is pure bulk and Physical Tanking (Even more so than Rock and Steel-types, nowadays). With access to powerful attacks, the ever-so-common Earthquake, and several moves that lower accuracy, the ground-type is also all about getting the upper hand against your opponent (In this sense, the Ground-type is the Physically-based counterpart to the Psychic-type, which explains Claydol's general versatility).
Anyway, enough with all of this Type Theory, let's get down to the question!
A definite option to consider for a Ground-type is Sandslash. With Base 100+ in both Attack and Defense, it makes a good Physical Tank (Not to mention that the pure Ground typing keeps it form inheriting any extra weaknesses). Sandslash can naturally utilize the Defense Curl / Rollout combo, which works wonders on it. It can also use Slash and Night Slash, for great High Critical Hit-Ratio attacks. Finally, the Rapid Spin support it provides is a real lifesaver in UU.
The Nido-Twins are pretty good too. Their incredibly wide movepools make up for their average stats. Plus, their typing isn't too bad either! The resistances they have also wonderfully compliment the ability to Tank.
For an offensive, Glass Cannon-style force, definitely choose Club-equipped Marowak over anything (even Garchomp!). Like the Light Ball on Pikachu, Marowak's Attack stat is doubled when it holds the Thick Club. This results in an enormous base 160 Attack stat! Pair that with STAB Earthquake, Stone Edge, Outrage (NO JOKE :D), and Fire Punch, and you've got one of the deadliest Physical Sweepers in the whole game, believe it or not. It's only problem is its low Speed, so get paralysis on the opponent first, or get some Baton Pass support in order to sweep cleanly.
Of course, nobody can forget Steelix! It's weaknesses drag it down a bit, but Base 200 Defense, and STAB Gyro Ball with base 30 Speed is just way too good to pass up!
You think I'd forget about Claydol? ;)
Claydol has low Attacking Stats, but with access to both a STAB Earthquake AND a STAB Psychic, is something to boast about. It can basically do whatever you need it to do! It can sweep, it can stall, it can annoy, it can set up Dual Screens, it can give Rapid Spin support, along with setting up its own hazards, and it can even be an Anti-Lead or a Suicide Lead. It's also got a pretty big movepool, so kudos to Claydol!
Whiscash is great too, and you can check my Blog for a good explanation as to why he's go great:
http://willspokeprofiles.blogspot.com/2010/07/poke-profile-whiscash.html
Flygon has Levitate, which is a good plus over Garchomp. It also has access to Fly, without being scared of Thunder, a slick, cool look (I, for one, think Garchomp looks like a total tool xD), and it has access to Bug Bite, which can throw off certain strategies played out by the opponent. Yep, Flygon's a great fly to stick in your opponent's soup!
The rest of the choices, like Swampert, Hippodon, Torterra, Gliscor, and Garchomp, are all obvious as to why you'd use them, but I figured I'd shed some light on the more underrated choices. These choices are all as viable as Garchomp, Hippodon, Gliscor, and Torterra, and NONE of these have a x4 weakness. =)
