I'd say Curses are fine to leave out for a first game, because their entire point is to not "add" to the gameplay, but to "subtract" from it, and that's just not the kind of new experience you want to give a new player-it's like telling the them the rules of football, handing them a ball, then tackling them to the ground.
I'd say the key things to introduce a new player two are the four types of plusses, attacks, reactions, and trashing. For that, the standard "First Game" set does well...
...but hey, we have all these sets out, let's see what else we can come up with.
First, the standard intro gets one thing right, definitely. Market and Village are fantastic introductions, both due to being so vanilla, but because of the different roles they serve. Both are spammable, but having each out leads to the introduction of the difference between +1 Action and +2 Action.
So, Market and Village.
We need an Attack, in order to teach that Dominion isn't just multiplayer Solitaire (just quite close to it, but with prettier pictures). Militia is a fine one, of course, but we're looking for something different. Spy is way too annoying, the trashing attacks don't sit right, and like hell we're inflicting something like Ghost Ship on a new player. So, I'd say it's between Cutpurse and Bureaucrat. Cutpurse is annoying in multiplayer games if everyone jumps on the train-by comparison, Bureaucrat is about as nonthreatening as Attacks get. In addition, Bureaucrat teaches another lesson: Silver is a nice card. An absolute beginner will look at it and think "Yay, I can deprive them of a card in hand!", transition to "eh, what does this card even do?", before having a "a-ha!" moment and realizing that every usage of Bureaucrat is like buying a Silver and putting it on top! And you still get to buy something else! Really, there's a lot to learn for a new player all in that one card.
So, we're up to Market, Village, and Bureaucrat.
We have an Attack, so we need a Reaction. The canon example is Moat, and indeed there's only a few Reactions that make a nice contrast with our Bureaucrat. Bureaucrat is way too weak to justify Moat, but Secret Chamber is plenty sucky to combine with it ("They play Bureaucrat, I can move all my victory cards to the top of my deck! Take that!"). It does lead into another thought, though...
Market, Village, Bureaucrat, and Secret Chamber.
Why not a three card combo? Village plus handsize reducers plus "draw up to" is a combo, and an easy enough one to see. In addition, a draw until card is a counter to Bureaucrat, also leading to the lesson that a counter to an attack needn't be blue with different words. So, we get Library, filling out our draw role quite well.
Market, Village, Bureaucrat, Secret Chamber, Library.
I'd say some trashing is in order. Remodel and Mine we're trying to avoid, and you know what? That's fine! We need a strong opener as well, something that dares a player "look at me, see what I can do for you" from the start of the game. An absolute new games player will skip over this one, but someone a bit more savvy will think "what is the point of this card? Let me try it." and they'll find out that opening Remake and getting rid of Copper and changing Estates into Silver is a nice springboard. Additionally, we score some interaction with Bureaucrat here...someone who's dedicated to Remaking their Estates will find that hard to do when they keep being sent to the top of their deck. It seems mean, but I can't really find it in myself to feel bad for buffing Bureaucrat. Remake is a powerful card that will reward a daring player, and a worthy lesson to be on this board.
So, we have Market, Village, Library, Secret Chamber, Bureaucrat, and Remake. We have four slots left, and many lessons to teach. We're skipping Cursegivers due to being newbie unfriendly (we want to encourage players to play cards, not disallow them from doing so), we don't want VP chips or mats or anything, and we don't want any Alchemy or on-gain. So, what else can we put down?
Well, alternate treasures, for one. Something a little shinier than Gold/Silver/Copper, to hint at the lesson that treasure is a thing to buy too and not just when you don't feel like buying your eighth Village. The best one for this is easily Royal Seal, as it's basically Silver with an upside, and it's a pretty neat effect, too!
With Royal Seal added, we have three fives, two fours, a three, and a two. We need a bit more low cost in there.
First Game has Workshop and Woodcutter. We already have a Workshop-like effect in Bureaucrat, so there's no need there. Woodcutter, though....there's really no replacing it, and it's an interesting contrast with Silver. The difference between the two is a lesson core to Dominion's gameplay, so we'll keep this one in.
As for other simple effects, I'm thinking Oasis. It's a simple enough card, it's a soft counter to Bureaucrat (see that again? It's our only attack, so if people just see the attack bit and buy and play it a bunch, it's best to have everything else respond to it to match how a new player would feel), and it also combos with Library. Additionally, it can be spammed like Market but without the annoying decision paralysis that comes from playing four Markets in a turn and having five buys with six coins to agonize over.
This leaves us one more slot to be filled, and I can't find myself not wanting another two cost. Our options are limited, though. Taking out complex, redundant, and new player unfriendly cards, we leave ourselves with Courtyard, Crossroads, and Duchess. Now, it's a little complex compared to the others, but it's not impossible to parse, and it resonates with all of our set's themes (Silvers are to be noticed and compared to, Bureaucrat is a thing, teach fundamental lessons), so I feel Duchess is a great card here. Additionally, new players will find it a nice moment of discovery when they realize they can "improve" their deck buy snagging a Duchess along with the Duchy compared to not doing so. Additionally, this will hopefully push newbies away from buying Estates willy nilly which is always a noble cause.
So, our final set of ten:
$5: Market, Library, Royal Seal
$4: Bureaucrat, Remake
$3: Woodcutter, Village, Oasis
$2: Secret Chamber, Duchess
Is this the absolute best board for a new player? Honestly, no, not really. The canonical "First Game" set is hard to top. However, I'd say this comes fairly close. It's a better bet if whoever you're teaching isn't afraid of a little complexity, as I feel my board sacrifices some of the initial elegance of "First Game" in exchange for more interaction between the cards and rewarding a keen eye to details.