I think many of the "classics" used a clever twist to a trivia contest (With Sale, you build up your bank to buy discounted gifts, Jeopardy you give the questions, TTD was just that...tic-tac-toe). But, the hook was making a pedestrian trivia game engaging, and those shows did that. Millionaire and The Challengers are really simple on paper (and honestly on TV too), but added their own twists that made them engaging.
On the other hand Free 4 All was the probably straightest Q & A I saw, and it made watching paint dry seem like a nailbiter.