A friend's? Or just something you encounter in the wild often? Well, I can understand either way.

[There's nothing like walking down a nice, calm path and sudden having a Graveller jump out and blow themselves up in one's face.]

I've never actually used one, but my mind immediately jumps to Wooper. They're part ground, which eliminates their weakness to electricity, though admittedly it does make grass-types even more effective against them. Plus, I'm pretty sure they can learn Earthquake, and that's always handy.