The Knickerbocker was a hit for a couple decades in the mid 19th Century and for some strange reason, it has been in the shadows since. It is one of my favorite summer drinks because it has a darker fruit profile than many rum cocktails and is a perfect balance of sweetness.
The raspberry is the key to the success of this drink and when they're in season, fresh berries make a great garnish. Also, if you like, Chambord makes a nice substitute for the syrup. You can also serve this in an old-fashioned glass over fresh ice.