Crabgrass

Crabgrass -- Digitaria sp is a fast growing, sprawling, clumping grass that grows all over everything. It is an annual, and tends to be out-competed by healthy lawns. Still, it tends to show up all over the place

Lawn Grass

Purely in terms of displacing native plants and exposing people to health hazards, nothing comes even close to lawn grass.  It takes a lot of work/energy/chemicals to successfully grow a lawn monocrop like this.  Don't get me wrong -- I think some lawn is great for playing on, but when it's grown just for the sake of having it and isn't ever touched, I don't appreciate it much.