Winter event 2023 is best for its nutcracker guardhouses, as a daily special. It's quite possible to save up event currency and than spend it on 15 guardhouses or as many guardhouses are needed to get 15.
For diamonds it could be somewhat interesting to spend on the 3k
offer. Granted it's either offering a golden upgrade for the chocolate factory or it's offering the winter23 grand building spawner. This is mostly relying on luck.
Finally the advent calendar has a few prizes of interest. Targeting those with master keys from DC could make this event offering a bit more decent rewards.
For the above, I would go as a f2p player. The main objective is getting those guardhouses to significantly cutting down on time. To getting those 15 guardhouses. Ironically secondary target would be this year's grand prize. It's a very decent unit supplier. With some okay buffs for today's golden lvl standard and somewhat decent resources.
For the whales, there's not that much to heavily invest in. If I had a massive surplus budget of money I could sink into the event, I would target the advent calendar. The 3x3 building is on its own quite strong. Offering great efficiency for buffs, production and very efficient fp production. Even outperforming the settlement's 1-time 4x4 fp building (30 fp/d at max lvl). Adding the 20%
from the forgotten temple and you've gotten an exceptionally efficient fp producer (4,7+
/tile). It's also serving as a weaker spawner for the winter piramide. Alongside that you're getting a golden chocolate factory upgrade, a golden selection kit for the winter piramide, FoE+ and a number of resources.
To me that appears as a solid deal. Although probably expensive due to the 50% spawn rate. It's probable that you'll end up in top1% depending on the server your playing on but even if your dropping down to silver, your still getting pretty much 2 golden chocolate factories and very solid rewards. The biggest risk however, might be a more efficient building showing up next year. Although this is a risk is limited. As competition of something even more efficient likely be something limited to 1 too. So, might be worth it to lay down a foundation for maximum fp efficiency.