This one is for Roadrunner.
Sadly, Scout is not a good card. In fact, Scout is a bad card. So bad that in most cases you would not even want to take a Scout if you would get it for free, and that is fatal for a card that is not even terminal. Why is it so bad, and are there cases where it's still worth picking one up? Let us find out!
Comboless Scout
First, we want to examine how Scout fares without the presence of cards that synergize with it in a meaningful way. Imagine for simplicity a Big Money deck that has Scout added to it. What Scout does in this case it make your next hand better by removing dead cards from it. However, this comes at the expense of making you're current turn worse. If you play Scout and pick up one green card, then sure one green card is missing from your next hand. But this turn, you are now stuck with a five-card hand that includes this green card. And who is to say that the turn you played Scout isn't the one where you could have needed the extra Copper instead?
So if Scout picks up 1 card on average, it's pretty much the same as not having Scout at all. He does nothing for you, but at least won't hurt you. In order to be actually helpful though, Scout will have to pick up more than one green card on average. For Scout to pick up 2 greens on average however, your deck has to be 50% green. This basically never happens in a normal game.
So Scout without support is bad. Normally, it will pick up less than 1 card on average, thus actively hurting your deck. For Scout to be worth it, we need cards that work well with it. The rest of this article will be spent looking for such interactions.
We can split Scout into three parts: He puts green into your hand, reorders the top of your deck and provides +1 action. Typically cards will combo with one of these abilities.
Getting green
As said before, as long as you put only one card on average into your hand, Scout does not do anything. If you want to utilize this ability, you need ways to ensure you get more green.
InheritanceWe start out with what is probably one of the strongest Scout combos. If you use Inheritance to transform your Estates into useful actions, then Scout actually picks up the good stuff. For maximum effect you want the inherited card to be nonterminal, so that you can really load up on them.
The problem with the combo is that you need to spike $7, and picking up a Scout will certainly not help you do that. So don't pick up Scout before you inherit. But after the Inheritance, inherited Estates themselves compete with Scout for the buy. You should always carefully evaluate whether another Estate is not better for your deck than the Scout.
Hybrid VPThis is what Scout was originally designed for. You still need a LOT of Hybrid VP for this to pay off. Even if you gain 8 Nobles, you would still need a deck with significantly less than 32 cards total to expect being able to draw more than one of them. And if you have such a small deck why did you pick up all those Nobles anyway?
The fact of the matter is, the hybrid VP we have mostly prefer large decks over small ones. The larger the deck, the smaller the green density, the worse the Scout.
Discard for benefitRemember how good it feels to discard a ton of actions to Storeroom and then redraw them with Scrying Pool? Can we make the same thing work with Scout and green cards?
Unfortunately no. Scout it just too bad at drawing. Even if you draw 4 green cards with Scout, playing a Storeroom afterwards, this will only net you $4. So you just spent $7 and two buys on something that in the best case is a terminal $4? Could have as well bought a Harvest.
I will say that Artificer is a much stronger discard-for-benefit card and might make this combo worth it. It's too early to tell.
RabbleRabble is countered by Scout; the Scout puts all the green into your hand. This is only worth it if your opponent plays multiple Rabbles per turn. Even then, you need to have the Scout in hand at the start of your turn. Scheme might make this possible. Wait, we're on a board with a Rabble engine and Scheme? Why are you wasting $4 on a Scout?
ApothecaryA cleverly timed Scout actually helps an Apothecary engine that starts choking on green. Apothecary leaves all the green on top of your deck; your Scout can clean it away. This is actually a decent combo.
Knowing the top of your deck
Sometimes Scout can be worth it even if he never picks up any green. Here are some examples.
MysticA Mystic-heavy deck benefits from Scout. Imagine your hand is 4 Mystics, Scout. Playing the Scout lets you draw four new cards. The catch is that 4 Mystics guarantee you to draw 2 cards anyway, so Scout effectively has about the power of a Lab here. It can be even better if you reveal more Mystics (or Scouts), which can continue drawing. For this to work, you only need one Mystic in hand. To pull the combo off, you need a high Mystic density though.
The Wishing Well combo is sadly too weak to ever matter.
Scrying PoolScrying Pool does two things with Scout: It effectively turns it into a cantrip because it is one more card that gets drawn with Scrying Pool, and if played before Scrying Pool it allows you to put that one non-action card on top to skip it with Pool.
Ruined Village anyone?
And then there are the oddball games where you just really need this +1 action, and don't have any other nonterminal lying around.
Peddler/ConspiratorJust wanna get a lot of actions in play? Well, Scout at least is nonterminal. This needs a lot of support in order to become a competitive strategy though.
PrinceScout just may be the only nonterminal, and you really want to start your turn with 2 actions. Very unlikely.
TokensSay what you want of Scout, at least it's spammable. Pathfinding actually turns it into a decent card. Of course most of the time you'd rather turn an already-decent card into an awesome card.
Conclusion
What have we learned? There are probably three major Scout combos to keep in mind: Inheritance, Apothecary, Mystic. If you see Scout without any of these cards, chances are you won't be any worse off ignoring it.