Bluegrass is so hard to pin down to one sound. Bluegrass is a pretty modern genre, the name coming from Bill Monore and the Blue Grass Boys, but the style and sound go way back the Scots/Irish immigrants and their celtic influences. Also heavily influencing bluegrass is the sound of black slave bands.
Inside of bluegrass you can break it down even further. Different regions of the South have their own particular sound. What you hear in the NC/TN mountains may not sound the same as what you hear in the Low Country of South Carolina.
I've never been able to name a favorite bluegrass band, though I do cherish the old-timers, Ralph Stanley, Bill Monroe, Del McCroury, Lester Flatt and Earl Scruggs. But I also love the influence that the rock generation had on Americana and bluegrass. Back in the 90s the combination of Jerry Garcia of the Grateful Dead with David Grisman and Tony Rice was outstanding. Garcia had even played blue grass back in the late 60s/early 70s with Old and in The Way.
Today, newer bands like Old Crow Medicine Show have given the old-timey sound an attitude and drive that has re-invigorated the music. The strangest newgrass band I've seen is more of a punkgrass band, The Avett Brothers. They play traditional instruments and will have ballads and tradtional type songs and then turn around and play wide ass open, jumping around and throwing banjos and stand-up basses around.