Alhambra (4) (3+1)
Colosseum (6)
Forbidden City (12)
Kilwa Kisiwani (12)
Temple of Artemis (17)
Terracotta Army (13) (16-3)
This will probably be my last vote since I don't want to bludgeon the horse, but I'll make my last argument and wax a bit philosophically. Firstly, I am not considering conquering the wonders, only building them. I thought that was the instruction of the OP, but I could have misinterpreted things. Obviously, if you conquer the wonders, you would only want to build wonders that benefit only or primarily the builder, such as terracotta army. If you assume you can't conquer the wonder, at least not in the immediate future (it's on another continent or something), and think purely in terms of what you would rather have in your empire, for me, I'll take Alhambra over Terracotta Army.
Why? I like things you can't do any other way. You can't get an extra military slot any other way. You can heal and promote your units. If you keep your units alive, they'll get promoted any way over the course of the game. You can use an apostle with chaplain promotion to expedite healing, or just fortify a few turns. There are useful military policy cards, to say nothing of Alhambra's other perks. Even having +1 amenity for garrisoned units, or +30% production to military/harbor districts, or +100% walls production, and later military science can be quite useful. If you are going pure units, you will probably want the cards that buff unit production, and won't have room for those other policies. Alhambra gives you more options and flexibility.
If you are on pangea, or facing only a few foes, I think the value of TA goes up, because you can snowball much more easily. If you are spread out and have lots of foes, I think the card is worth more, especially on higher difficulties. Having some extra promoted units won't seal the deal when the opponent has cavalry and culrassiers to your crossbows. Just my 2 cents!
Love the diversity of thought and opinion.