My 'commonest' drink is scotch and soda over ice. Having said that, I also enjoy the occasional gin and tonic.
When doing something on purpose requiring actual preparation, I like my version of a French 75. A French 75 is cognac or brandy with [lemon in the original, but I don't like lemon and substitute] lime juice, then fill up the champagne flute with champagne. All components chilled beforehand.
For an after-dinner or bedtime drink, I mix one large cup of decaff coffee (I don't sleep well) with one jigger of Irish Creme and one jigger of Drambuie. It's hard on my blood sugar but pretty easy on my mouth. I call it a 'Claymore'.