Chashio is pretty much hitting the nail on the head here. The variety in brushes and needs is so great, there's really little I can recommend. For example, in the current world I'm working on, I want my land features to look a little "spotty", like some flecks of paint bled or blobbed on the canvas a bit. I'm using the brush "chalk02" in the GIMP for this, which I've never used until now. Had no need.

And heck, just looking over more of the default brushes, I'm thinking "vegetation02" will be a great start for my forests. *shrug* There's just so many options here that the best way to find what works for you is to open a new file and mess around until something resonates with you. Sorry I can't give anything more specific!