Concrete will be best, but you will have to properly compact the soil, lay the proper sub-base for drainage, have a vapor barrier, and pour the correct concrete mix atop the above. If not, you may end-up with a floor that looks great but slowly allows moisture to penetrate your new concrete flooring and rust your appliances. In other words, seek and install a properly engineered floor to save you years of potential grieve.

If you went with gravel you'd have severe rusting issues with the washer/dryer due to the ground moisture coming up through the gravel.
Concrete would be the best, making sure a drain is installed in the floor too.