The Colosseum is just so absurdly good- I do wonder how many cities the devs balanced it around. I think it is probably the single best wonder in the game. Even if it only gave amenities, it would be the best. The culture and loyalty are sprinkles on top.
As someone with an engineering background, I am physically unable to resist getting Mausoleum + James Watt + <3 Nikola Tesla <3 and Joseph Paxton every game. 2 charges on Watt means every factory is giving off +7 production. That's 4 more hammers in every city. Tesla lets you go beyond that and get a nice big 12 tile, +8 power plant. Paxton lets you set up a 12 tile uber EC with +7 amenities to every city in range. It is just SO GOOD that I can't honestly see why they applied it to engineers. (Interestingly, a Watt factory gives +7 aura. A city with Watt + Tesla will recieve +11 from its own factory, but other cities will still only get +7 from the factory aura. It is very odd, I wonder how it's coded!)
The +card slot wonders are also all amazing, especially now that we have legacy, dark age, and much better diplomatic cards. Before R&F potala palace was... meh.
While I don't mind having these always strong wonders in game, I think some are a little too good in every situation (Big ben, cough cough.) I really love how they've got a whole cast of very situational wonders- Petra, St Basil's, Temple of Artemis, Huey are all wonders you may have no use building, but the right city will get massive benefits. I really like this because then you aren't just doing the same rush every game.