The 1990 Aqueous Test cards have actually been fairly flooding eBay in the last couple of years. However, despite that, the prices have gone through the roof.
A few that I've had absolutely no luck with for my Dave Winfield Collection are:
1985 Subway Disc - An MSA product of which I've never even caught a glimpse.
1986 Sportflics Jumbo Disc - I question SCD's checklist, as all I ever see are the Mantle, Clemens, Mattingly and Gooden. I've never caught a whiff of the Winfield disc or coupon that allegedly exists.
1989 Donruss Blue Chips - I'm not even sure what these were supposed to be. The checklist mirrors the 12-card Grand Slammers set, and from posts on FreedomCardBoard's forum, there is even a Leaf version of the set. I doubt I will ever see this card. I've seen two colors of the Leaf version of the Canseco, so it's also possible (though perhaps unlikely) these all exist in the 5 different colors like the Grand Slammers set.
My only real success with rare modern cards was this complete 1982 FBI box:
