For the first several sets the best you could possibly hope for would be to buy a nice 5-cost card on your opening turn (Maybe a Lab or a Market or something).
Then Guilds came along and introduced Baker which gave everyone an extra $1 that could be used at any time (including your starting turn). So now it was possible to buy a first turn Gold. Neat.
Adventures added Events and included the Borrow event, which when coupled with the coffers from Baker opened up the possibility of a $7 buy on the first turn (in case you really wanted one of those 7-cost Prosperity cards).
Next up, Nocturne added Heirlooms including Pooka/Cursed Gold. Now an opening 4 copper + Cursed Gold + Coffers + Borrow meant that not only is an opening Province possible, but you could even swing an opening Platinum. However, the coveted first-turn Colony was still a bit out of reach.
Until Menagerie added the Desperation event. Desperation, combined with everything else mentioned above makes an $11 Colony buy first turn a possibility if the stars align just right. No one could expect much more than that for an opening turn, right?
Plunder says, "hold my beer". How does an Inherited Capital in the opening turn sound?
However, this introduces a bit of complexity to the equation. If we are abiding by the standard rule of no more than two unbound landscapes per game, then the addition of Inherited, kicks Borrow out of the loop. Additionally, the biggest potential single buy in the game is Dominate at $14, but that's another event, so if we're going to go for that, it kicks Desperation out of the running as well, at which point we get:
Inherited Capital (6) + Cursed Gold (3) + Copper x3 (3) + Coffers from Baker (1) = $13. Just shy of buying Dominate (unless I missed some other mechanic that could give us 1 more coin in the opening turn without using up an event/landmark/project/way/trait slot).
So if we're not buying Dominate, we can put Desperation back in which gives us $15 to spend on an opening turn. Add Tracker/Pouch to the mix to give us an extra buy and the question becomes, what do we spend that $15 on to get the "Biggest" opening buy?
If we're just going for the most victory points, we can't do double province, but Colony/Island is probably better anyway for the same VP value plus the ability to get useless cards out of your way.
If we're going for biggest treasure buy, we could go Platinum/Gold for $15.
I'm sure there are a lot more strategically effective opening turns than those, so what would you do given just the right kingdom? And did I miss some other card that could be inherited instead of Capital for a bigger effect?
How about Masterpiece and 12 silvers? That sounds like a great opening turn.