But since the UN wasn't an option... I voted Hagia Sophia, because I can get the same result with Serfdom (minus the culture and GPP) and I don't need to compete with other civs to build serfdom. I hardly notice the difference of +50% worker productivity anyway.
If you don't start in an era where it starts unlocked, you should virtually never be in serfdom.
Also, UN candidates are 1) whoever built it and 2) next highest person in population other than guy who built it. Considering you can win with it, it has no place on a list like this

.
Cristo Redentor would be infinitely more broken if it showed up earlier in the game.
Fixed

. Combined with the "influence civic" and "influence religion" missions, a 1 turn crossover in civic switch w/o anarchy would allow for some of the more flagrant abuses of diplomacy the civilization series has ever seen

. Religious group hug? Naaaah. I'm taoist now...and so are you! But next turn, I'm buddhist again, and Izzy joins the tag team DoW...
I built the Space Elevator in my last game. I thought it was a fair investment, although I think I did have an engineer waiting for it. Robotics didn't take too long to research, and had some benefit anyway, and I'd only built the Apollo Program and no SS parts prior to the Elevator, so the discount was well worth it in shields. To be fair, I only had two cities, and nearly all my production was in one of them. And I still lost the game, since I was too far behind when I built the Elevator to catch up. But I don't think it's as bad as it's made out to be.
It is hands down the worst wonder in the game (in BTS) and researching robotics to attain it almost guaranteed you a slower finish.
A good way to beef up Space Elevator may be allow you to select and finish 2 space parts instantly. Kind of like Oracle and Liberalism for space ship parts.
Either that, or make it available earlier on and from techs that are actually required for a space victory. Even very production-weak empires tend to wind up with a tech bottleneck (I've had empires of nothing but cottage-carpet still have the bottleneck in tech, not

), so any effort to get robotics is an instant loss of time. You'd need to get robotics from the internet, have the AI not build SE themselves in that time span, have two engineers handy, AND not have them provide more benefit by simply doing a golden age...and even then the SE only saves 1-2 turns!
Mausoleum of Maussollos for me. I almost never use Golden Ages, (Other than with the Taj Mahal) and don't care much for Great Artists, so it's basically a culture building for me.
Not using extremely powerful features does not rationally qualify them as the worst in the game.
Regarding the Space Elevator on the other hand, it may be a questionable wonder using default settings. However, with the settings I use - K-Mod, Immortal, no tech trading/brokering; I like being behind and having to struggle - the AIs are often ahead of me in the late game. In these cases I am forced to start a war, sometimes against two huge and advanced Civs simultaneously, in order to deny them their victory. I will desperately be churning out units and have no time to build any spaceship parts. Mech Infantry is a very useful unit for late-game wars so I will tech robotics anyway.
When I do eventually find time to build my spaceship, having the space elevator is very helpful. I may have denied one Civ their victory by destroying their capital after they lanched their ship, or razed one of their legendary cities. But meanwhile other Civs may be striving for victory, and having the SE may make the difference between winning or losing.
Did kmod change the techs required for it? If not, then building it was still an awful idea. If you have the oomph to beat down a space-launch AI and teched robotics, you were not making a serious play at space. It's very easy to build a ship and launch it when everybody else who could realistically do so is dead. Use those hammers on a couple nukes or something and win faster.
In fact, the different settings used by players may be the reason why the benefits of the SE are so heavily disputed; for some settings it's useless, for others it's a very beneficial wonder.
Show me a game where you haven't researched robotics yet and are actually attempting a space win, and I'll show you a game where pursuing the space elevator slows down your win.
Long ago I made a challenge thread: Show me a game where SE makes your time faster rather than slower. Years later, not one person has managed to do it. "depends on settings" my foot. Maybe if you start in future era and don't just kill everybody.