Whiskey Sour is my current fave. I make it with roughly:
1/3 lemon juice
1/3 simple syrup
1/3 Maker's Mark
Shaken, poured over ice in a lowball glass, and add 1 or 2 maraschino cherries.
With only 3 ingredients, you'd think it'd be almost foolproof, but it's amazing how many bad ones I've had in cocktail lounges.