Yikes? Out of those above?
EU it must be, because all the other fill me with cold dread about what will become. Pan-christiania pretty much fill the same space as pan-islamia with a different flag. Equality to all, except for those who do not conform to our faith! All religions are equally valid, but ours is correct. Anybody who wishes can get elected leader, but only white/Arab Christian/Muslim males born and bred in the US or Britain/Middle East actually stand a chance. If they're filthy, stinking rich, that is.
India has enough problems governing itself as it is, and shows a truly troubling lack of concern for its poor.
China is a rapidly metamorphosing giant. What will come out, dictatorship, anarchy or capitalism? Nobody truly knows.
Aliens? Well... I kinda like the idea of humanity ruling its own destiny.
Liechtenstein? Not likely. They're capable enough when it comes to running their own little state, but it's unlikely they'll be able to adapt to their new responsibilities.
The USA? Well, it is the prototype democracy of the modern world... And, like all prototypes, functions far less well than the updated versions. The two-party system pretty much ensures power stays in one place, and it is far too corruptible and exploitable. It also fails to take responsibility for its downtrodden masses (Ironic, isn't it?), and has far too much of a gung-ho, "We Wanna Be Heroes", "Let's Go Get'Em, Boys", agressive attitude to actually be a viable option. It needs to mellow pretty seriously to get my vote.
The EU is still in it's early stages, and, as such, suffers from birth pangs. The system looks to be rather easily refined, however, and the entire mess seems to be developing in the right direction.
My true answer, however, is "nobody". Power corrupts; absolute power corrupts absolutely. The best thing might be a system like the one we have, except without any wars or famines or epidemics. My second choice would be if all the nations on Earth joined into their local alliances and those alliance joined to form a United Earth. The other runner-up is if all nation-states die quietly without being noticed, and people manage without nay artificial divisions.