I'm going with Excadrill for a few reasons. First, it's Ground AND Steel, which makes it tough against a lot of types. Second, you see Sandshrew a lot while you rarely see Drilbur. It's the whole rare factor. Third, if trained right, an Excadrill can be devastating. I haven't really used a Sandslash in battle yet. I preferred using a Marowak than a Sandslash.
My last reason, it was on my main Pokemon team on White when I kicked Team Plasma's butt.