I say Spain as they are Europen champions and reached the final.
Barcelona have played in 7 Champions league/European Cup finals. THey reached the finals in 1961,1986,1992,1994,2006,2009 and 2011.
Manchester united have reached Champions League 5 times. They reached in 1968,1999,2008,2009 and 2011.
Arsenal have never won the European Cup or Champions League. Their best performance was in 2006 when they reached the final, but was beaten by Barcelona 2-1.
Arsenal have NEVER won the European Champions League. They have come close many times, most recently when they reached the final in 2006.
Arsenal have only reached the semi finals of the Champions league twice. They reached the stage in 2006 and 2009.
Manchester United have reached 4 Champions League Finals. They won the tournament in 1998-1999 and 2007-2008. They lost the finals in 2008-2009 and 2010-2011. They also reached a final, and won it, when the tournament was still the European Cup. This was in the season 1967-1968.
Chelsea have never auctually won the Eruopean Champions League, however this year 2012 they have reached the finals just like they did 2008 when they lost against Manchester United during the penalty shoot out.
They reached the semi final in 2001.
We Are The Champions reached Number 1 in the UK, France and many other countries.
they reached the final but lost to Barcelona
Chelsea have failed to win the Champion League title upto this year, even though they have failed they have reached the final many times.