I can't really pick one, because there are many I consider to be crucial, or near crucial. I tend to want nearly all of them. Instead, I'll list the ones I don't care about:
Pyramids (yeah, I know, I'm DEFINITELY in the minority here - I'm shooting for the Great Library)
Great Lighthouse (unless, of course, I need it to make contact with others - then it's a good one)
Great Wall - 2nd most useless "wonder" in the game
Oracle - expires, relatively quickly
Hanging Gardens - see Oracle
Leo's - I'll take it, if I can get it, but I'm much more concerned with other things, usually (Sun Tzu, Sistine, et al)
Shake's - nice culture and all, but...
Magellan - Nice to have, but certainly not necessary
Longevity - wow, it makes my cities grow too much so they starve back down! Most useless wonder out there.
Cure for Cancer - nice, but I rarely take the detour to genetics anymore.
Manhattan Project - This should be a minor wonder, that all civs must build prior to building nukes. As it is, why would I build it? It just allows others w/the tech & resources to build nukes earlier than they otherwise could. No way, I make them build it.
Just my thoughts.
-Arrian