Ground is the type I use the least, because their designs are usually a bit boring to me... The type itself does work for me though (and also, Flygon). Similarly to Ground, Fighting types don't often appeal to me. Most seem too humanoid, and too physically oriented, so their attacks don't keep me entertained.
Lastly, Poison. Although I love poisoning opponents, and most designs aren't bad at all, most of the times I give Poison types a try they tend to bore me really fast and I end up replacing them. I can't exactly say what it is about them...