The best liquid to use in tiramisu is dark rum.
Dark rum is the most common liquor used in tiramisu.
However you can also use other types of liquor in tiramisu such as Marsala wine, brandy, rum, Amaretto, Frangelico and even Sambuca.
Marsala wine is a traditional choice for use in tiramisu.
Marsala wine is a sweet and fortified wine which adds a light and fragrant note to the tiramisu.
Marsala wine tastes like nutty brown sugar and also pairs well with coffee.
Dark rum is another popular choice for tiramisu.
Brandy is a popular choice for boozy desserts and can add dept to tiramisu.
Kahlua can also be used in tiramisu and is a coffee flavored liquor which can also complement the coffee and chocolate in tiramisu.
Frangelico is another good choice of liquor for tiramisu and is a hazelnut flavored liquor which can complement the coffee and the chocolate in tiramisu.
Sambuca is also another good choice of liquor for tiramisu and can cut through the richness of tiramisu.
You can also use Madeira or VinSanto in Tiramisu.