Either Mike's Chapel or Hoover. No rioting in the cities and high production plus pollution reduction... it's almost all we need.
Plus some wonders work... wonderfully in conjunction - Colossus+Copernicus+Newton+Shakespeare = SSC whcih handles ALL of science with proper trade routes.
United Nations - priceless later in the game, when the AI only thinks how to tear you apart.
King Richard's - yeah, I love the shield boost, especially in large cities, best if I have enough cash and time to build in SSC
Other than that:
Marco's - very useful, although I don't build it on smaller maps.
Leo's Workshop to upgrade warriors all the way up to riflemen, as well as settlers to engineers.
Adam Smith's is useful on lower difficulty levels, but it can be a decent boost if built early on deity.
SETI - although at later stage in the game, my SSC is usually doing pretty good job of providing me with techs every 2-3 turns. Even less with well placed vans.
Apollo is a must in landing games so it just has to be built, especially if you're in the tech lead, to start the SS early on.
Bach's Cathedral - very useful if you can get the tech early on, and can afford an off path tech at that time. Best to discover the tech once you have a fully working SSC.
Magellan's - Sometimes, when the distance really counts for better trade routes. Most of the time though I rely on close enemy civs that are not too far away. After that - colonies + airports work nicely, although there's nothing as good as a decent ship chain.
I almost never build:
Eiffel Tower (useless... it really is)
Great Wall (uhm... no)
Great Library - if I can afford it, I build it and then disband the city. Perfect if the AI builds it and I find the city and raze it till it vanishes

Oracle (expires to early, plus I don't usually build temples unless its the only building left)
Statue of Liberty (once in demo, I don't switch till I discover everything,
otherwise oedo for revolutions earlier)
Hanging Gardens (expire too early, plus I usually go for early colossus + Cope)
Sun Tzu (Leo upgrades everything anyway and I lose vet status)
Manhattan (unless my ship has landed and I feel like playing some more)
Lighthouse (maybe on island maps)
Darwin's (let the AI build it and then steal the techs =D)
Pyramids (demo or rep +super growth make granaries obsolote)
Women's Suffrage (although it might be useful in conquest games)
Cure for cancer - also useful to control unhappines if you have to slide the lux bars further to the left, to catch up. Other than that, better spend the production for a few more vans and transports to establish good trade routes. AI rarely builds it so it's another plus