Pyras/Zeus probably...
(if you're so free at that point in the game chances are your NC is not going to be up early anyway)
Pyras cost exactly two workers, but building simply two workers never fail (and the first worker can forest-chop out the second one earlier), and pyras sometimes are gone early (but sometimes are built t200)
+15% against cities is nice, but hardly game breaking. If you're going to capture a city chances are you're going to capture it. With or without the bonus.
I don't get it: even on Deity, you quickly know if pyra are gone or not.
If not, why bothering building 2 workers when you can have a free +25 and a GE point at the same time?
In my Deity games (and I play lib 90%) I never failled to get it: either be gone or I got it, but never in the middle.