The question of "most political" is a tricky one to answer. How would one measure that? You could argue that if a show makes a political statement, it's just as political as any other show that makes a political statement. I suppose some shows will have subtle, very subtextual political statements, requiring thorough analysis to pick apart. Or some shows might include political statements, while the show as a whole focuses on non-political issues. You also may see cases where the show is about politics, or political figures, but doesn't necessary make a strong political statement about the subject matter - it might just present the facts of the situation.
But if I'm interpreting the OP's question correctly, it's not necessarily about which one is the "most overtly political" (i.e "which overtly political show is, in itself, presenting the most political subject matter or viewpoint?"
but rather which one is the "most overtly political" in the sense of "of all the musicals that deal with politics in any way, which one does it most overtly." It's a slight semantical difference in how you interpret the question, but I think the latter is easier to measure, and it seems like it might have been what the OP was asking.
The trouble is there are a lot of musicals that present political messages pretty overtly. So we sort of have to split hairs a little bit if you want to argue which one is the most overtly political. So I guess you just have to ask "in which musical is the political message the LEAST veiled by metaphor or allegory?" and then "of the shows that have no metaphor or allegory, which ones speak most directly to what's happening right now?" and then, "of the shows that speak directly to what's happening right now, which of them presents an issue that is most prominently in the zeitgeist and the social discourse at the time it's produced?"
I would say something like Soft Power ticks most of those boxes, probably moreso than Urinetown IMO.
Urinetown is hard, because it does deal with the political issues in a way that is overt and direct within the context of the story. The characters have to fight over which political ideology serves the people best. And yet, we as a society are not currently facing a mass water-shortage that would result in the scenario presented in the show (*knock on wood*). Our debate over socialism vs. capitalism manifests more directly in other issues like health care, food resources, housing, etc. So you could argue that the imaginary dystopia - though directly responding to political issues - is not as overt as other shows, because it has that one thin layer of separation from reality.
