Eh, this kind of depends on what you're into. California, Oregon, Washington, Idaho, Colorado, Utah, and parts of Montana, Arizona, and Wyoming are all great for running. Anything in the south is going to be sub-par due to summer weather. Can't speak too much about Florida as I've spent minimal time there. Anything along the east coast is likely to be lousy (spent some time in NYC, DC, and North Carolina and it SUCKED for running compared to out West). States like Oklahoma, Kansas, Nebraska, Iowa, etc are all decent, but not a lot of elevation or mountains, and you often have to deal with either heat or wind, which bumps these states out of the top ten. The Northeast might be good along with right around the appalachian mountains, but I haven't spent any time there so I can't comment. Western South Dakota is actually very nice. Can't imagine anything around the great lakes would be decent.
Out of the places I've been, I'd say it's a matter of personal preference between CA, OR, WA, UT, ID, CO, and maybe MT and AZ.