I will pick Bulgaria not because they are the best team among the selections. They have the easiest group. They are the only team with some chance of making to the 2nd round. The other teams all got at least two teams who are title contenders in the group. Having said that, Bulgaria probably finishes last in their group while Greece could finish 3rd.
I had to choose among Bulgaria and Switzerland (Despite Being Greek! ) because I think our group is to tough this time. I certainly HOPE we will do good but I will also be happy with good football win or lose. I think that Switzerland are a possible surprise in tha making if they are also a bit lucky.
Greece will go the farthest of any of these teams. They've proven they can beat Spain, and then all they'd have to do would be to beat Russia, and they would be through to the next round.
Bulgaria. Don't even think that they are weaker than Sweden or Denmark. And Italy will struggle against them, too. They've the best chances to proceed among these four, IMO.