Silver
Really? Without Silver, it would be a LOT harder to get up to $5 and $6 cards, and you'd need a lot of Gold to buy Provinces!
Okay, so in the spirit of making this a teachable moment...
Yes, an early Silver (or something that gives you $2) makes it rather easier to get $5 cards, but if you look through the game logs of top players or watch the streams, you will find that, surprisingly often, the Top 100 only buy the one, and they might even trash it or pass it later on after it's served its purpose.
And there are surprisingly many games when top players never buy (or even gain) one at all, and yet these are often games that are over in like 12 or 13 turns. Once you figure out how this works, your rating cannot help but go up a lot.
I try not to but too many Silvers when I'm playing with an Engine, but you really need a 'slighshot' card like Moneylender or Baron to get you up to 6 on turn 3 so that you can jump right to Gold. With BM, however, a Silver is almost always a good buy.
See, when playing an engine, you usually want up to 1 silver, just to help you reach $5 on your second shuffle. And when you do get "slinged" to $5 or $6, the card you want to be buying is not gold, but some powerful engine piece.
You want to get your engine up and running as fast as possible, and Gold will not help there (it may even hinder you, by making your engine pieces less likely to connect). Instead, $5/6 is price level that contains some pretty powerful engine cards you want to get ASAP (of course, depending on a board): cards that let you draw (Wharf, Journeyman, Stables...), slow down your opponent (Mountebank, Witch, Goons...), are powerful trashers (Upgrade, Junk Dealer...), offer some other benefits or combine any of the above (Margrave, Minion, Governor...). In engines, all of these are more important than an early Gold.
Okay, you're probably right, thanks!
Think of it this way, a Smithy at game opening is worth $2.1. A Smithy with one Silver in deck is worth $2.45. Smithy only gets more value when you have villages and can chain them. $5 actions almost always start worth a lot more than a Smithy.
In general, Engines tend to mean buying zero - three silvers.
1. Zero - I can generate engine components without Silver (e.g. Chap/Workshop/Remodel can quickly get villages, draw and then Remodel $4's into Golds if you can do that quicker than building in draw to use Silver over gold
2. One - hit $5 a bit easier, something good to open at $4.
3. Two - nothing good to open sub $5 and a need to hit $5 or up early
4. Three - no copper trashing engine - $7 from coppers, $6 from Silvers, and $3 from
a gold gives us $16 - enough for two provinces. Ideally, you'd have 7 coppers and 3 golds (save a card), but normally you don't hit $5 often enough to skip the silvers entirely.
Much more than that is certainly possible (e.g. University/Margrave/Chapel means 8 Silvers are most efficient than than 5 golds and a copper), but you need a really good reason to go above three silvers in an engine - most boards don't need to go there.