Specialists contribute the most to GP generation, then Wonders, then National Wonders. For reference, each specialist produces 3 GPP every turn, no wonder in the game gives you more than a raw 2 GPP, no NW gives you more than 1.
So this means that if you're looking for great people, specialists will almost always be more helpful than wonders. There are obvious exceptions, of course. For example, the Great Library gives you two free specialists, so it's the best of both worlds. But generally speaking, if you're running Scientists in a city with a bunch of Great Artist wonders (Sistine Chapel, Parthenon, National Epic, etc) odds are much higher that you'll spawn a GS than a GA.
You're right about Engineers. I think the game was specifically designed to limit GEs in the early turns - you can't generate them at all until Metal Casting unless you built the Mids, and even with MC, Forges only give you one Engineer apiece. Compare that to two Scientists from a Library, two Merchants from a Marketplace, and unlimited Scientists, Merchants and Artist from CS.
Here's the real kicker: 90% of the time, a Great Scientist or Great Merchant will be much more valuable than a Great Engineer anyway. GEs are good, don't get me wrong, but Great Scientists bulb most of the path to Liberalism while also giving you Academies in your research cities, while Great Merchants can be cashed in to either upgrade your troops (not terribly cost-efficient but sometimes necessary) or to run at 100% science (recommended).
Insta-building a wonder is nice, and they have some decent bulbs, but GEs for the most part just aren't quite as useful. I mostly use them to snag those wonders that I don't really care about but are nice to have (if I pop one in the Middle Ages, for example, I might trade for Engineering and use him to rush-build Notre Dame, a good-but-not-great wonder that I'd otherwise skip).