Today I'd go with iPad (and before you write Flash, see http://itunes.apple.com/us/app/mafia-wars-shakedown-by-zynga/id465811033?mt=8 ) and consider the Google Nexus, Samsung's latest and of course the Kindle Fire.
Is what app any good? You didn't name it.
I am wanting to buy a tablet but I will need to be able to play mafia wars by zynga on it. Which tablets do you think would be best for this. I believe Mafia wars needs Flash. I heard about an app for ipad that will let you play flash games, Is it any good?