My 18's are wrapped in 285/60-18 tires. They are 31.5" and I ran them before my lift. My wheels are Granite Alloys, tires are Sumitomo HTR Sports (all-season)
I went with those tires because they were the cheapest and I have another set of wheels with mounted M/T's and my 18's will most likely never leave the pavement. BFG makes a 285/65-18 All-Terrain that is 32.8" tall. It all depends if you plan on offroading with your 18's or not.
What rims do you have? Many 18" wheels are 9 or 10" wide and would therefore limit the overall diameter you can use. For example I had 18"x9" moto metals and fit a 32"x11" (275/65R18) stock with no issues. I also know that 285/60R18's will fit also. So as you go wider you have to go with a smaller diameter.