The Allman Brothers Band stands the test of time better than the others, in my humble opinion.
Allman Brothers were great, no doubt, but I think the death of Duane Allman (which by the way, Ronnie focused the words in "Free Bird" around) really hindered the band. I think Gregg Allman did a great job carrying on the band, but I'm not hugely sold on Betts. If you can think that they had "Whipping Post," "Dreams," "In Memory of Elizabeth Reed" (which Dickey Betts wrote), "Midnight Rider" (a Gregg Allman penned tune).
Now "Little Martha," "Melissa," "Ain't Wasting Time No More," "Win, Lose or Draw," "Ramblin' Man," were all gems (my personal favorite Allman Brothers tune is "Seven Tunes")
But could you imagine what they'd be like WITH Duane still in the group? I'm not sure you can compare the Allman Brothers to Lynyrd Skynyrd to the Marshall Tucker Band anyway. The Allman Brothers (who before were known as the Allman Joys), were HUGE into blues music. They loved the feeling of soul and blues music and it reflected in their style. The Marshall Tucker Band were much more into country style music. Lynyrd Skynyrd was more a hard rock/heavy metal mixed with a tad country and at times blues. It's still quite sad to see major pieces in music lose their lives too early. Hank Williams Sr., Duane Allman, Elvis, Ronnie Van Zant, Steve Gaines, Toy Caldwell, John Lennon... I mean they say everything happens for a reason in life...I'm lost on that reason. Granted, with Skynyrd, Ronnie wanted to branch off and become a country artist and manage the band while his youngest brother, Johnny eventually took over. Johnny did, however, we lost an absolute legend...and to me, the single most greatest lyricist and vocalist in music.