Couldn't be iOS 7. It's either iOS 5 or 6. Since it originally came out with iOS 3, I'd say the two generations rule would probably apply and iOS 5 would be the cutoff. But they introduced iOS 4 a couple months after the iPad was introduced, so it's doubtful it could be called a full generation. But, they released iOS 4 for the iPad in November, so although April to November isn't a whole year, it still could count as a generation. Therefore, I'd wager that iOS 5 is going to be last for the first iPad.