I'm a Johnny-come-lately to soda fountain drinks, but I'm gonna go ahead and say that The Franklin Fountain in Philadelphia's Old City neighborhood makes the best root beer float anywhere.
Those older than me would probably have more accurate accounts of what a soda fountain was originally like, but my first (and only) association with a place like this is "The Candyman Can" scene in Willy Wonka and the Chocolate Factory. Franklin doesn't have a wall of sweets behind the counter (though they do stock a few old-school taffies and gums), but other throwback elements are there: the staff in soda-jerk whites, the checkered floor, the pull-down stools, and supposedly one of this country's oldest countertop draft lamp for dispensing soda water, which sibling owners Eric and Ryan Berley restored to full working condition when they opened the shop in the summer of 2004.
Part of the lengthy soda fountain history that the shop details on its website recounts the early days of ice cream floats. One story goes that in 1874 a Philadelphia native named Robert M. Green happened upon the idea of plopping a ball of ice cream into a glass of soda water when he ran out of sweet cream (typically added to soft drinks) and bummed a scoop from a nearby vendor. Another claims that Green himself had sampled the combination even before that and introduced it to Philadelphians that year. Either way, the idea stuck. Sample the real deal at The Franklin Fountain and you'll see why.
As they did with their other soft drinks (including their perfect New York Chocolate Egg Cream: milk, Fox's chocolate syrup, soda water, and a pretzel rod as a salty stirring tool), the Berleys studied up on soda chemistry and perfected their own root beer. Besides its robust spice, the quality that makes it stand out from, say, Barq's or Mug, is its viscosity. Unlike most sodas that feel like sweetened soda water, this one has considerable body, reminding you that there's syrup in there. As for the ice cream, it's by far the richest, most intensely flavorful vanilla ice cream I've ever had. The dairy itself tastes like it has more sweetness and mouth-coating fat than most versions, and, for once, all those black vanilla bean specks aren't just for show.