Puke. I was thinking Cleveland, Columbus, or Cincinnati would be good choices because they're the political epicenter of America, but then you mentioned Portman and I realized you're right... the little turd WOULD most likely try to take an "active role" if it was in his home state, and he'd probably even want to introduce the nominee or give a speech about "small businesses" or something (he'd still slip some pro-gay rights reference into the speech anyway). Ohio is a no go.
Looking at the remaining options, the most logical choice for me is Denver because we need that state to stop voting RAT in Presidential elections, and we have a good chance at some statewide pickups this year to build ground for 2016. I know the RATs have been to Denver before, and fairly recently, but looking at the list of GOP conventions, I don't think they've ever given it a shot: http://en.wikipedia.org/wiki/List_of_Republican_National_Conventions
The freeper suggestion of holding it Detroit so we could inflitrate deep inside the enemy's camp and show America what one-party RAT rule has done is intriguing irony, but do you really want to torture the delegates by forcing them to get hotels there? Putting in Denver after the RATs celebrated Obama there is a big enough middle finger to them.
I hope the RNC will finally lose Colin Powell's invite, but I won't hold my breathe.
Another one I have considered is Iowa. The rats should NOT have won that last time. They lost North Carolina and Indiana from the first go-around. Iowa is a largely rural state. There is no excuse to keep losing it.
But I think turning Colorado around is a priority.