I've been grilling chicken (and all that entails) the past couple of hours so don't hold me to these in the morning. But on first thought:
70's = 76 Blue Devils
80's = 85 Cadets (Though 89 Cadets is my favorite)
90's = 91 Star Watched that one on TV with a bunch of Cadets, Suncoasters, and various others and we were all amazed. Beautiful show! And about half a dozen from 92 would make a short list of my favorites.
2000's = Pick you favorite from the Cav's. They weren't my cup of tea (loved their shows from the 80's and 90's) but they were so darn good. I should also check the years for some of Crown's stuff mid to late 00's. Wow!
I was supposed to split up the 2000's I guess, but meh. I'd end up choosing Cadets 2001 and one of those great Crown shows and shafting the Cavs all together, and they were too fantastic for that.
2010's = I'll be a homer and go with Cadets 2014. Not well designed to the sheets, or those that love still love sitting down low, but I thought that was the Cadets doing Americana like only they can do. Performed lights out. Blue Coats could easily reach that last year, and any show by BD. Loved SCV's Les Mis too.