Pina Colada
Rum, pineapple and cream of coconut. The confusion between cream of coconut and coconut cream ruins more of these than anything else.
Mai Tai
Two rums, lime, orgeat and curaçao. No pineapple juice, no grenadine, no umbrella required.
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.