The Grateful Dead were an American treasure. They did more for and to music than any band before or after. Anyone who says, "sure they were popular but not great" has never given them the time or heard them live.
Forget about their studio albums, although "American Beauty" and "Working Man's Dead", are pieces of art, and listen to a live show. Try "One From the Vault", then tell me that they were not great. And, that is just the tip of the iceberg.
I should add that, "Wake of the Flood, "Terrapin Station", and "Blues for Allah" were pretty darn good studio albums but those songs played live became much, much, more.
The Dead's music spanned almost every genre. They were not a one trick pony and continued to evolve over time. They had great song writing, excellent lyrics, could tell a story and could play the heck out of a song on any given night. I don't think any other band before them or after them compares.
They certainly were a "band beyond description"! As the saying goes, you might not think they were the best at what they did but they were the only ones that did what they did! Thus, making them the greatest American rock band every!