What makes the "top" show to you? Different things to different people. For me, it is the # of quality koi entered, not just the # of koi. It is history, it is the judges, it is how well organized. It is the trophies...for some

Nice trophies are cool.
It is hard in this country to have a truly National Show, as in Japan, UK, or Holland, among other places. The distances and resulting logistics are too great.
Some top US shows historically have included, but are most definately not limited to, ZNA Gardena, San Diego, ZNA SoCal, MAKC Koi America, San Antonio. A GC from any of those shows carries some significant weight with many long time koi keepers. There are several other "up and coming" shows, depending on who you talk to. To me, these may not have the history of the above group, but are heading in that direction. Florida, Washington, Oregon, SF Bay Area, Chicago, OKC, Denver (ok, I am biased

) come to mind. I know there are others.
Many Shows have a dealer division or competition. Some think that dealers are already competing in the hobbyist division, but that is another thread. (Ducks

)