Granny Smiths are probably the most tart generally available apple, but any cooking apple in general is usually sharper. You might also like McIntosh or even P

Granny Smiths are probably the most tart generally available apple, but any cooking apple in general is usually sharper. You might also like McIntosh or even Pink Lady. You could also go down the cider apple route or even crabapples if you really want, though they often tend to be more bitter than tart.