What I mean is, in practice it doesn't matter which card is the "best" combo with scout, since in practice it should never be acquired unless there are 3 other cards available that combo with it.
It doesn't matter how many cards that "combo" with it there are. No interaction alone is enough to make Scout better than nothing outside of extremely fortunate draws that never actually happen in practice, and it doesn't get better than nothing just because you have multiple of those interactions available. You might want to acquire it because it's an Action card (on a Vineyard board), because it's named Scout (on a Fairgrounds board) or because it costs $4 (on an Upgrade variant board) though.
I meant don't get it unless you have already made an engine that has a good amount of at least 3 of the cards that are in the poll of this thread. And even in that special case I wouldn't go out of my way to get scout, I would only get it if I had workshop / ironworks etc in my hand, unless, like you said, there was vineyards.
What i am trying to say is, if you already have a strong engine with a healthy number of cards of multiple types, all of which synergize with scout, is it worth acquiring to add an extra oomph to your already strong engine using cards such as nobles / highway, apothecary, etc.? Absolutely it is, if it's convenient to do so and as long as you are not doing so at the expense of acquiring other more useful cards.
I have definitely made crossroads / apothecary driven engines that benefited from the addition of scout IN THE LATER PART OF THE GAME.. Such games are rare but they exist
