Easy one. It was Jimi Hendrix. The Allman Brothers were still up and coming at the time (only first album released), although they had become well known in Atlanta.

Certainly Jimi was the biggest act, however I don't think they followed a real order of performance, this is what I found, but the performers are not in the order they played, looks like Hendrix closed on the 4th........Peace.....joe

Atlanta International Pop Festival 1970 Setlists
JUL 3 1970
Date
Friday, July 3, 1970 - Monday, July 6, 1970
Venue
Middle Georgia Raceway, Byron, GA, USA
So far there are setlists of 32 gigs.

Friday, July 3, 1970
Ballin’ Jack
Captain Beefheart & His Magic Band
Goose Creek Symphony
Gypsy
John Sebastian
Mountain
Savage Grace
The Allman Brothers Band
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Saturday, July 4, 1970
B.B. King
Bloodrock
Cactus
Cat Mother and the All Night Newsboys
Jimi Hendrix
Lee Michaels
Poco
Rare Earth
Rotary Connection
The Allman Brothers Band
The Chambers Brothers
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Sunday, July 5, 1970
Grand Funk Railroad
Hampton Grease Band
Hedge & Donna
It’s a Beautiful Day
Johnny Winter And
Mott the Hoople
RIG
Richie Havens
Spirit
Ten Years After
Terry Reid
The Allman Brothers Band
The Bob Seger System
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[Edited on 1/13/2019 by crazyjoe]

Jimi was the headliner at every festival he played. Later that summer he headlined the Isle of Wight Festival, which also featured The Doors, The Who, Joan Baez, Leonard Cohen, Jethro Tull, Richie Havens, Joni Mitchell, Chicago, Ten Years After, and many others.

Thanks for feedback