Gin and Tonic - not really a cocktail - Tanqueray, lots of ice and lime juice squeezed in first and a slice on top.
Of conventional ones I could go for a Margarita....made properly, 3 shots Tequila - a good blanco, one of Cointreau, two of lime juice (freshly squeezed) on the rocks or crushed....don't need the salt rim. Certainly no sweeteners as I saw one barman in the States put in them after a round of gold - we refused to pay for or drink them.