I'll echo what others have said, that some of these cards absolutely don't deserve to be in this discussion. I have an Effect With of 1.58 for Adventurer, and it has my 11th highest win rate with of any card, directly behind Counting House at 10. Contraband is my #5, behind only Followers, Trusty Steed, Princess and Sea Hag. Contraband is definitely the most out of place card on this list, but that's been defended pretty well already, so I'll go ahead and stand up for Adventurer.
Adventurer may not be the absolute cornerstone of any one strategy, but that's true of many cards that wouldn't even be considered for this list. It's a card that can push a pretty solid deck over the edge, it's much, much better in Colony games, and it's often a useful addition to engines on boards with light trashing that want to draw their whole deck every turn but can't quite make it. It pairs excellently with Throne Room and King's Court, which, sure, most cards do, but not nearly all of them. It's worth picking over gold just once at some point on boards with good trashing that result in decks light on treasure, and useful on almost any board in which you're draining victory card piles. It's absolutely underrated in otherwise-mirror matches that end in duchy-dancing.
It's even a usually worthwhile buy in games with heavy cursing/no trashing - throw it in a kingdom with Sea Hag and Colony, for instance, and you should pretty quickly understand it's utility. Yes, in that situation it's going to be worth $2 instead of $3+ much of the time, but it'll also serve as a Loan-esque defense against Sea Hag and a perfectly reasonable cycler. For this reason it's also perfectly worthwhile against attacks like Rabble, Fortune Teller and Ghost Ship.
And then there's Bank. Adventurer has the problem of being compared unfavorably to Venture, but if Venture's not around Adventurer is still capable of pulling off many of the same tricks, and has the advantage of the aforementioned King's Court - a KC'd Adventurer can be an absolute godsend, especially with extra buys.
And how about Vineyards? Did you put together a crazy deck bloated with action cards only to start losing the vineyard race because you can't cycle to your potions fast enough? Well, have I got a card for you... Horn of Plenty is another good partner - did you build an engine designed to get to 8 unique cards and trash Horns for Provinces, only to not pull your horns at the right time? Alternately, would the horns of plenty you did draw get there if you could just find that one Silver you have tucked away? Really, any time there's a specific special treasure card in your deck that needs to be played often, there aren't many cards that will be better at digging for it.
Obviously some of these are fringe cases. I don't want to imply that it's a great card by any means. The cost is obviously the big issue, and it's true that it would probably work just fine at $5, where it would be a much stronger $5 than many people realize, but still not broken. I wouldn't even put it at the bottom of the $6+ card list.
I didn't get into the game until after Prosperity came out, so I can't really say for sure, but my guess is that the presence of Venture in the game has likely ruined people's ability to perceive and play this card correctly, and created an unfair reverse cellar/warehouse style comparison (where the more expensive card is considered directly inferior) that doesn't really apply to these two. They certainly do a great many things similarly, and Venture is quite a bit better at some of them, but that's not really the point - Venture is an excellent BM card, and Adventurer is an underrated engine cog that is directly useful in more situations than people care to realize. It simply does too many things at least reasonably well to be the very worst card in the game.