Pina Colada
Rum, pineapple and cream of coconut. The confusion between cream of coconut and coconut cream ruins more of these than anything else.
Mojito
Rum, lime, mint and soda — built in the glass, and ruined more often by over-muddling than by anything else.
Daiquiri
Rum, lime and sugar. The simplest real test of whether a bar knows what it is doing.