As Barcelona lifted the Champions League trophy at Wembley back in May, the rest of the football world could have been forgiven for questioning whether they would ever topple Josep Guardiola’s slick squad. After all, even the harshest judge would concede that Barcelona, in Lionel Messi, Xavi and Andres Iniesta, boast three of the top five players on the planet.

The task of ousting the Spanish giants got even harder over the summer as Guardiola signed Cesc Fabregas and Alexis Sanchez, just adding to the attacking options at his disposal. Slight question marks remain over goalkeeper Victor Valdes and the defensive credentials of the back four, not to mention their ability to replace like-for-like, with Javier Mascherano often filling in at centre back – but Barcelona press so effectively that these are usually moot points.

And so, though no team has recorded back-to-back triumphs since the competition entered the Champions League era, Barcelona are understandably favourites to retain the trophy as the start of this season’s competition approaches.

But this is just the kind of challenge that the likes of Jose Mourinho and Sir Alex Ferguson will relish. Real Madrid and Manchester United – along with Manchester City – lead the pack looking to dethrone Barcelona and all three teams have the attacking threats to back up their claims.

Mourinho’s men wasted little time making their feelings known during the Spanish Supercup against Barcelona as the second leg descended into a major scuffle. As was the case last year, much will depend on Cristiano Ronaldo, who has taken the team on his back time and again, and the ever-improving Mesut Ozil. Real Madrid would also receive a huge boost if Kaka could turn back the clock and rediscover the form that made him arguably the world’s most coveted player five years ago. The nine-times winners will reach the knockout rounds comfortably, despite drawing Lyon and Ajax in Group D, but then the tougher challenges will come thick and fast as Mourinho seeks the club’s tenth crown.

Meanwhile, Manchester United’s injection of youth over the summer strengthens their credentials. Ashley Young has settled instantly while Wayne Rooney looks on course for the best year of his career. The centre of midfield remains a concern, however, with Ferguson finding neither the destructive ball-winner nor the creative spark that would be so priceless in Europe. Progress from a weak Group C, containing Benfica, Basel and Otelul Galati, will be a formality and Ferguson will hope to be blessed with the kind of route to the final that the English champions enjoyed last season.

Rivals Manchester City are in the Champions League for the first time in their history, but were less fortunate with their draw, ending up in an uber-competitive Group A. Villarreal, Napoli and, particularly, Bayern Munich will provide stern tests for Roberto Mancini’s high-flying squad, though it is hard to see the likes of Sergio Aguero, David Silva and Samir Nasri failing to create a bagful of chances for the in-form Edin Dzeko. While missing out on the knockout rounds would be classed as a huge failure, going as far as the semi-finals at the first attempt might be beyond a team that is still gelling.

Outside these four clubs, the biggest threats are likely to come from Andre Villas-Boas’ Chelsea, who can still call on an array of proven, experienced stars and hope to revive the out-of-sorts Fernando Torres, and Bayern Munich, who have developed an exuberant, youthful core led by Thomas Müller and Bastian Schweinsteiger.

2010 winners Inter have slipped down the pecking order since Mourinho’s departure, but may yet force their way back into contention if the likes of Wesley Sneijder and Giampaolo Pazzini hit top form. The same could be said of AC Milan, however Arsenal, so often an attractive outside bet, endured such a wretched summer that even reaching the knockout rounds is far from a foregone conclusion for Arsene Wenger’s men.

Spare a thought too for the competition’s newcomers this year, taking their place in the Champions League group stage for the first time. While City will have little trouble adjusting as they take their bow, it is hard to see fairytale stories for the likes of Czech side Viktoria Plzen and Romanians Otelul Galati, but the experience will be priceless for their players.

On paper, the 2011/12 Champions League winner seems destined to come from the trio of Barcelona, Real Madrid and Manchester United, but the beauty of the competition is that nothing can be taken for granted in the knockout rounds – as shown by FC Porto and Monaco appearing in the 2004 final and Inter earning a surprise victory under Mourinho. The only guarantee is that club football’s premier tournament will serve up its usual blend of thrills, spills and skills this season. Sit back and enjoy.