First we have my favorite base for your Zombie Apocalypse, they call it "ash waste"... I call it "ashy color skin if you dont move your ass around those zombies"
This is a nice base for those who dont have the time or wants to mess with making vegatative bases. I would suggest getting some.
My next two favorites for modern games are Urban rubble and Industrial. They both have a tendacy to be "busy" but since they are made from resin its easy to file down the parts you dont care for.
Round 25mm Industrial (5)
These are good for any old floors or non modern games... I have scratch built some of these in the past... lets just say its easier on your sanity to go ahead and buy these.
Round 25mm Flagstone (5)
Now for some stuff a bit out of the ordinary. The deck plates could be used in modern games but I suspect they are designed for sci-fi games. I wouldnt mind having a few. The skull bases are very nice... for those times when you need to stand on your enemies remains.
Round 25mm Deck Plate (5)
These are even more out there for our types of gaming but the Fractal bases are just what you need when your team goes to the harsher climates. The warp flame to me look more like water... so its in this capacity that I would use them.
25mm Fractal Round Bases (5)
All of these fine bases are 25mm and run around $4.00 USD. You can also get them in larger sizes and they do come in square and in bevelled 30mm sizes.
Enjoy!