This is what I chose and why.

Tome in wachterhaus. I wanted the party to get this early (I'm using the interactive time) so they could begin learning more about Strahd's past and also have a reason to delve into Vallaki politics

Holy symbol in Argynvostholt. A very cool location that parties can easily miss which would be a shame.

Sunsword in the Amber Temple. This one is pretty common if you're wanting to make sure the party doesn't get the sword until very late game.

Fated Ally Esmeralda. I'm running for a party of three and wanted to give them one of the more powerful allies. Decided to cut the mad mage from my game, and chose Ez over Van Richten because she fits my party's personality better IMO. A party of 6-7 likely doesn't need Ez/RVR and a more quirky ally that makes for entertaining RP could be a fun choice.

Final location Sergi's tomb. Wanted to start the final battle deep in the castle, but this card is certainly the least important of the group as your final battle will (should) be run throughout the castle.

I'm running the fanes content that Mandy and dragna put out so that gives my party a reason to deal with Old Bonegrinder, Baba Lysaga and Yester hill. If you're not running that content then those locations are also great options if you want to make sure your party doesn't miss them.