The following is to the best of my knowledge and may well be outdated; the optical media industry moves *very* fast when it wants to.
"Authentic" TDKs are made in the USA, possibly other places too, but you might want to avoid big spindles marked "Made In Taiwan", as those are probably made by Ritek; not that I've had any problem with my Ritek-made TDKs - they're supposedly held to higher QA standards than Ritek's normal lines, though I'm not sure I believe that...
I don't think there is such a thing as "authentic Memorex"; AFAIK they've always sold rebranded discs from other manufacturers.
As for Imation, I'm not sure they make their own discs either; the only ones on
CD Media World (though these are definitely not the only ones out; CD Media World's info is often outdated) are rebranded CMC Magnetics (a so-so company from what I've heard) and Taiyo Yuden (excellent).