I voted for the Eiffel Tower.
There's an smallest benefit from it, and AI benefits zero from it. Let them cope with the stupid tower
Others discussed;
- Great Library; can be essential, yet can be negative. If you're playing with an single city and 6 other civs, you will benefit from their knowledge. On the other hand, there's another AI issue...the AI knows when you get certain advancement, regardless if they even made contact with you. If you haven't discovered certain advancement that grants a wonder, AI has, AI won't build the wonder for a number of turns. When you discover it, they'll start building. So, if one AI civilization has Astronomy, second one discoveres Astronomy, you get it from the GL, you're in the middle of KRC building, they start building the Observatory. Pretty horsehockey situation.
Oracle - possibly needed when you're playing on the higher levels with increased unhappyness. Makes extra happy people until you're ready for Colloseums, without it you can be forced to remove workers to avoid disorder.
Lighthouse - on the large, archipelago based maps with small land mass, essential. Easy colonization of new islands with triremes, good for early trade, and if AI gets it, they'll start spamming the sea with their boats, they'll find your little island and harass you in the B.C. era (i'm playing MPG so they're not very friendly).
Manhattan Project - on smaller maps, if you have enough capacity to build 20 nukes in 7-8 turns, great for conquest. If the AI hasn't discovered rocketry yet, you can just nuk'em all and take down their cities. Complete ownage in one turn.
Darwin's Voyage - extra advancement, maybe a waste of resources if you're already ahead of them, but like i said, if they build it, they'll narrow the gap.
Statue Of Liberty - for me, this is a great wonder. I'm playing Democracy as soon as i get it. This is a scenario that i witnessed too many times...i'm at stable peace treaty with AI civ. He builds a city close to mine, overlapping squares. Then, he forces me to remove all my border guards, fortified units, because we now "share" the teritorry. Then, he uses the 2 moving points unit, first he places him on my square, that square gets out of my control, he puts his workers on it, and removes the unit. Steals my square. If i have the statue, i hit the revolution, smash his city, and it's only one turn of anarchy...nothing to worry about.
There's an smallest benefit from it, and AI benefits zero from it. Let them cope with the stupid tower

Others discussed;
- Great Library; can be essential, yet can be negative. If you're playing with an single city and 6 other civs, you will benefit from their knowledge. On the other hand, there's another AI issue...the AI knows when you get certain advancement, regardless if they even made contact with you. If you haven't discovered certain advancement that grants a wonder, AI has, AI won't build the wonder for a number of turns. When you discover it, they'll start building. So, if one AI civilization has Astronomy, second one discoveres Astronomy, you get it from the GL, you're in the middle of KRC building, they start building the Observatory. Pretty horsehockey situation.
Oracle - possibly needed when you're playing on the higher levels with increased unhappyness. Makes extra happy people until you're ready for Colloseums, without it you can be forced to remove workers to avoid disorder.
Lighthouse - on the large, archipelago based maps with small land mass, essential. Easy colonization of new islands with triremes, good for early trade, and if AI gets it, they'll start spamming the sea with their boats, they'll find your little island and harass you in the B.C. era (i'm playing MPG so they're not very friendly).
Manhattan Project - on smaller maps, if you have enough capacity to build 20 nukes in 7-8 turns, great for conquest. If the AI hasn't discovered rocketry yet, you can just nuk'em all and take down their cities. Complete ownage in one turn.
Darwin's Voyage - extra advancement, maybe a waste of resources if you're already ahead of them, but like i said, if they build it, they'll narrow the gap.
Statue Of Liberty - for me, this is a great wonder. I'm playing Democracy as soon as i get it. This is a scenario that i witnessed too many times...i'm at stable peace treaty with AI civ. He builds a city close to mine, overlapping squares. Then, he forces me to remove all my border guards, fortified units, because we now "share" the teritorry. Then, he uses the 2 moving points unit, first he places him on my square, that square gets out of my control, he puts his workers on it, and removes the unit. Steals my square. If i have the statue, i hit the revolution, smash his city, and it's only one turn of anarchy...nothing to worry about.