My preferences usually go to older, pre-WWII erotica, or from immediately afterward, if possible amateur material like home made nude photos, drawings and illustrations. Underground illicit publications are also nice, old under-the-counter magazines and such. And early sex mags from the late 1960s to 1970s have a great appeal and charm. They are usually individualistic, publishers were still trying out all kinds of stuff, trying out new looks and layouts, formulas.
After the 1990s and 2000, publishing sort of went downhill, everything became formulaic, identical one to the other, less inspired and original. About the only plus-point is that many new books on the subject of old erotica were published, amateur collections reproduced, old photos republished in anthologies etc.
I've been lucky to still be able to find genuine old material: amateur and semi-professional drawings and under-the-counter publications from before WWII or directly afterward.
But as the years go by, older material gets harder to find. there is a very intensive hunt for old erotica by collectors worldwide. Prices become very high, things are directly snapped up, collectors make deals with old book dealers to hold erotic material back for them, bought sight unseen. Old photos command high prices.
And the times are becoming more prudish by the year. Formerly in Brussels secondhand bookshops run by Moroccans for instance, there were sections of hard core porn mags for sale. Now, aside from one or two stores, no one at all still sells such material. It's been collectively decided somehow that it is now out of bounds.