I want a challenge for the first game, so I'll dive straight into emperor.
I'm hoping the Civ V AI won't cheat at higher levels, but will get bonuses that the human player does not. Alternatively the higher difficulty levels could handicap the human player instead of giving the AI a bonus. I would be surprised if they've managed to make an AI that could fight on equal ground against a good human opponent with no advantages.
I divide "cheating" and bonuses something like this:
AI cheating is stuff that the human player can't do, such as seeing through fog of war, instant wonders, influencing combat odds, knowing what the human is researching and producing etc.
Bonuses mean stuff like 20% extra research, extra production, less unhappiness, starting with extra units etc.
I'm ok with the 2nd kind of way of helping the AI out, but I hope they've managed to make something that will be challenging without the first. I would be very wary if they think they've created something that can go against a human opponent with no bonuses though. Napoleon's way of leading the french in battle at the Immortal level in the livestream certainly didn't seem good enough to beat a good human player in tactical combat let alone anything else. The AIs of today aren't anywhere near the level of matching a human in a game as complex as Civilization.