Here are five more classics that came up in conversation when debating this topic, but didn't find a champion among the panel.
Orval. Not only is this Trappist beer considered the quintessential Belgian pale ale, but it is also the direct inspiration for Goose Island's Matilda, and should thus be credited with introducing a huge number of drinkers to traditional Belgian brewing.
Samuel Smith Taddy Caster Porter. This superb beer flies the flag for one of the most foundational beer styles: the English porter, which defined British brewing in the 18th and 19th centuries. As craft brewers around the world tinker with porters (largely revived by Americans), the likes of Taddy Caster and Fuller's Porter give them a standard to work from.
Dogfish Head 60, 90, and 120 Minute IPAs. There's no doubt that Dogfish Head is one of the most influential modern breweries in the world, and founder Sam Calagione told us himself that the 60 Minute IPA is one of the most important brews of his career. By giving the brawny West Coast IPA tradition an East Coast spin, it played a key role in setting off the coast-to-coast arms race for hop bomps—and so-called "extreme beers"—that would come to define American brewing. The 60 Minute is also, simply put, one of the most common on-ramps for newcomers to craft brewing and IPAs.
Ommegang Hennepin. You could make an argument for almost any of the Ommegang beers being on this list, as the brewery—established by Don Feinberg in Cooperstown, NY in 1997—was one of the first Belgian-style breweries in America. It introduced countless Americans to Belgian styles like saison (Hennepin is considered the first American saison) and inspired a generation of craft brewers to take Belgian brewing and give it a stateside twist. By collaborating with Belgian breweries (and, eventually, getting bought out by Brouwerij Duvel Moortgat, makers of Duvel), it's also been influential in creating a two-way conversation between the countries.
Hitachino Nest Red Rice Ale. Firstly, it's remarkable how many non-beer drinkers recognize Hitachino over what many nerds would consider the classics of British, German, and Belgian brewing. However, its significance goes deeper than mere branding: 1) Its use of rice has opened many people's eyes to the potential of alternative grains beyond barley and wheat. 2) As a beer from Japan that it is not a generic lager, it has increased awareness that brewing history is not the sole province of Western Europe, and there is in fact a world tradition worthy of celebration.