I was wondering what materials are best used to make high quality bases. I'm looking for your typical GW grass/dirt base like you would see on Rohan or Uruk Hai and Also the cave style you would see on moria goblins.

I always add a bit of rough sand followed by fine sand on a base. Then after it dried up I paint over it with some drybrush methods.
In the end you add some bushes or shrub on top.

There are also basing ways to get a piece of slate and put it on top of the base.

With half of my models I also use some cork on top of a base to heighten it a bit.

In general I got all the stuff from here: http://www.greenstuffworld.com/en/49-basing-materials

Maybe you like my bases or not, but here you can see what I've done: viewtopic.php?f=50&t=31894
(I suggest looking at the bottom of the page to see my most recent)