As soon as I saw the topic, I immediately said "Treasure Hunt" since that ran for four years and no one has heard of it that I've talked to, but everyone seems greatly intrigued by the premise. But, I'm going to strongly second "Scrabble" (seriously; 6 years in its original run plus 4 years of reruns on USA...haven't seen it rerun or rebooted since!) and "Shop Til You Drop".
I would have also answered "Split Second" had GSN not revived it, though I wonder how many people know it's a revival?
Oh, and on the kids' side, "Fun House". Understandably, "Double Dare" gets all the love, but "Fun House" was its own parallel phenomenon that practically no one I know outside of fandom remembers, though I find it somewhat funny that its spiritual successor gets a ton of love ("Legends").
Anthony