There aren't many cards I consider absolute "must haves," but for years right near the top of my very short list were the 1998 Donruss Crusade cards. In particular I loved the red version, which fit great since the color had been introduced into the Pirates color scheme in 1997. The down side? The reds are the rarest version at only 25 copies.
I have the reds for both Ron Wright and one-time Pirates shortstop of the future turned utility player Abraham Nunez. Perhaps not the most spectacular names, but the cards look absolutely amazing in person. One way or another, the cards are on a refractor type stock that I haven't seen duplicated since.
Panini tried to latch on to the popularity of the original Crusade cards over the last couple years. The cards are nice, but they don't hold a candle to the originals.