My two cents:
1. Millionaire: It's elegant in its simplicity, especially in that early era before we got eleventy gazillion celebrity specials and gimmicks. Just captivating television, through and through.
(Significant gap here between Millionaire and the rest of the list.)
2. Greed: Ultimately I appreciate it because it put its own grittier spin on the million-dollar craze, and did pretty well with it. But yeesh, the team-slayer survey questions are something else. Truth be told, it got to be a matter of which one of these I was least uncomfortable putting in second, and this did it.
3. Winning Lines: If it were just the Wonderwall, this'd be second place for me, easy. The overall format, I think, works a lot better in the original British version where 1) the contestants were folks who got lucky enough to have their phone numbers match last week's picks as opposed to that being a home sweepstakes afterthought and 2) you're playing for a trip around the world -- in that version the first couple rounds are a good buildup for that kind of prize. Here? It doesn't feel like enough when you're going to be playing for a million dollars at the end of the road. That said, Dick Clark is great, and the show is still a decent package.
4. Twenty-One: This is the one of the bunch I wish I could like more than I do. I love the atmosphere of those early episodes, especially with the live orchestra. I feel like the raw format can have plenty of drama! As it is, they sucked out so much of the originality out of it with the multiple choice questions and the second chance option (and then by getting rid of the live music and switching to bland over-the-top orchestral tracks). More than that, though, I loathe the strike system because of all the anticlimaxes it led to. You shouldn't be able to win $200K just because your opponent got cocky and struck out in two questions -- at least you should have to work for it a bit. And Perfect 21 is an afterthought all around.
5. Chance of a Lifetime: If ever there was "we have Millionaire at home," this is it.