I base this decision on context. My favorites are Great Library when many civs, Lighthouse/Magellan if LOTS of islands, Leonardo’s if at all possible. Hoover Dam and Women’s suffrage are indispensible, and Statue of Liberty and UN are quite useful too. All civ II, if that’s not obvious.
[even further hijack]
Yeah, “Universal” Suffrage in Civ III is lame (effects limited to one city). But Sun Tzu’s Art of War kicks ass!
[/even further hijack]