The teams mentioned by Catenaccio are the obvious candidates... But I would say both Milan and Bayern have better chances than Juve. It sounds pessimistic, but on one side, Milan will be boasting great confidence; they are the reigning champions and their team has barely changed - they will know in their minds that they are VERY capable of winning the CL.
Bayern are really hurt by last year's disaster... I haven't seen them play in a while, but an offensive lineup including Makaay, Elber, Pizarro and Ballack is quite intimidating... adding to that the fact that they, together with Juve, are one of the world's most solid sides.
We can't rule out Real at all... Los Galacticos have all the pressure, as has been the case in the last 3-4 years or so. Queiroz doesn't convince me, personally. I know he worked at Manutd, but other than that, I don't think he has coached a top team on the levels of Juve, Milan, Manutd, Barca, etc. Correct me if I'm wrong. Plus, their defence is looking worse than ever - Hierro is gone, and I don't believe Pavon can take on the responsibility of replacing him. According to the latest reports Valdano has made it clear that IF they sign a defender, he must be cheap and young... maybe a good prospect for the future, but almost certainly not an advantage for the coming season. The thing is Real may receive 4 goals, but they can easily score 5.
Manutd have shown great preseason form... Veron's departure might not affect them THAT much, in my opinion... He wasn't on the pitch against Juve on the 2nd half and Manutd still played better and even scored. But in the end, it was only preseason. however, they have still got some great players in hand; there's not only Giggs, ruud and Scholes. I think Solskjaer will rise up to their level this season; Beck's and Veron's departure will free him some space - he certainly has the talent. There's also Butt and Keane... authentic midfield lions.
Last but not least Juve. We've strengthened our side, no doubt about that. What worries me most, however, is that we don't have a clear goalscorer - many would say it's Del Piero, but over 30% of his goals are penalty kicks. We can't depend on dives our fouls within the opponent's area to score goals. Trezz is clearly off form, so I feel we are currently relying on the odd goals from a defender or a midfielder. That's why most of the team scored at least a goal last season. It's good to have many players capable of scoring, but it's also important to have one SURE scorer. It's what ruud is for Manutd, Ronaldo for Real, and Pizarro for Bayern.
Other than that, I'm fairly happy with Juve. I expect us in the semis at least.
Other teams I think are worth keeping an eye on are the New Chelsea (Duff and Veron will add A LOT), Inter and Real Sociedad (they are an extremely compact team... even thouogh they've almost certainly lost their captain, De Pedro, the major palyers are still there: Kovacevic, Nihat, etc, etc. - however the lack of experience plays against them).