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.
I quite enjoy the Mausoleum at Halicarnassus. I find retiring the Great Admirals a second time useful, but I use it even more with my Great Engineers. I have found it very useful with the engineers who boost wonder production and help space races. I have used it with Carl Sagan to win games very quickly.
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.