Which currency is the best to bring?

If you are coming from Germany then the Euro would probably be the best for you. It is easy to convert to CUCs in cuba.

US$ is the WORST currency to bring. It is hit with an additional 10% fee.

Apart from Havana, Santiago, provincial capitals, and a few well-touristed towns like Varedero, Vinales, Trinidad, and Baracoa, you'll need mainly Cuban pesos converibles, known as CUC. May as well get them right there in the airport upon your arrival (upstairs, after you've passed through customs).

If you really plan to spend a lot of time in the countryside or smaller towns, you might want $20 or so of Cuba's other currency, the moneda nacional, or CUP. There's not a lot you can buy for this (it is intended for Cubans, to allow them to buy at prices subsidized by the government, as some small compensation for their very low wages.) However, it's handy to have for the ferry across Havana Harbour, city buses, produce from street vendors, and fruit from the odd vendors along the highway out in the countryside.

You'll get a better exchange rate for CUCs with Euros. I never even saw a single CUP, nor did I see any way to convert currency to CUPs. I question whether or not a Cuban would take those from a tourist, seeing how the CUCs are worth so much more.

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<hr>I question whether or not a Cuban would take those from a tourist<hr></blockquote>

There are several products and services you are supposed to buy only in CUP in Cuba, even if you are a visiting foreigner. CUPs can be exchanged even at the airport, as well as in some CADECAS, most banks and even some hotels.