First if in direct sun this top can become very hot so if that is the case than look for a lighter color and under trees will have a tendency to always have dirt and leaves that can stain on them. As far as conventional outdoor stones are concerned (e.g. Bluestone, Crab Orchard, Limestone, etc) they are great looking, but very porous and will stain especially with grease from grilling. Marble can be used especially if you want a special patina feel because they will dull out quickly and if honed they will need sealing often. Granite probably the best natural stone choice, but some of the lighter choices will be more porous and need to be sealed often. Granite will also loose some of its shine over time. Quartz countertop materials (e.g. Silestone, Cambria, Ceaserstone, etc) unless UV protected cannot be used outdoors and currently I don’t know of any that can. There is a newer man made product which is my favorite choice today for an outdoor kitchen. This is Denton; it is a completely impervious material, super hard, will not stain and is very durable. Currently it only comes in 2cm thick and about a dozen colors. All in all try to pick a material that suits your style and surroundings and has a pattern or swirl that might show less dirt because even the morning dew will add dirt marks.