Well this is how I think of it. I either pick the type that has a majority of (catchable) Pokemon I like or I pick a type on a whim. With Ground types, it was the second one, and I'm loving it!
I think other cool types would be - Dragon (transfer over a Bagon or Dratini), Ice (transfer over a Eevee for Glaceon or a Swinub), and Normal types (diversity!).
You're solo-running Krabby/Kingler, right? I always liked Kingler, I had one on my other SS save file.