Big Ben = 31 - 3 = 28 Everyone can use more Builder charges (for a long time), more amenities (always) and a Wildcard slot (always). Everyone can use another Economic slot since those cards are the strongest overall, but not everyone will benefit enough from doubling Gold to make the production cost worthwhile. You don't have the luxury to hoard Gold every match. And 1760 production isn't that much of a deal for effectively just an Economic slot when you are close to unlocking Fascism/Communism/Demo and are basically 2 techs away from Power Plants which you probably want ASAP. Of course it's a great wonder as it can allow you to mass upgrade your Knights into Tanks or straight up buy 2 or 3 Tank Armies soon, or adopt Democracy for a short time, dump your huge money pool into Great People and continue with Communism. Still, it's more situational than Colosso, Pyramids and Forbidden.
Colosseum = 24 +1 = 25 At this point I am really hesitant which of the 4 relevant ones are the strongest. I'm voting for Colosso because I don't believe it's any weaker than Forbidden or Pyramids, and having +10-+12 amenities more in your empire since Classical Era seems like a much better deal for me than Economic slot and doubling of a yield I might have not the luxury to hoard, at Industrial Era.
Forbidden City = 31
Great Pyramids = 32
Goodnight, Sweet Potala, you landed in top #5, high enough given current overall strength of Diplo cards.
Moderator Action: Fixed Big Ben & Colosseum's vote count -- Jon