I am a big fan of the French 75, which is itself a riff on the Collins, the grandfather of all long drinks. Here are a couple of random, experimental variations which proved too tasty not to write down.
1 3/4 oz Plymouth gin (London Dy would work well too)
1/2 oz Aperol
1/2 oz real pomegranate grenadine
1/2 oz lemon juice
1 3/4 oz aquavit (I used North Shore, which is recommended if you can find it, Linie if you cannot)
3/4 oz Cocchi Americano
1/2 oz citrus syrup
1/3 oz orange juice
1/3 oz lemon juice
1 3/4 oz cognac
1/2 oz Rothman & Winter apricot liqueur
3/4 oz lemon juice
For all of the above: shake and strain over fresh rocks in a tall glass, then top with 1 oz Champagne or Cava.