UNQUESTIONABLE
Frankie Beverly and Maze - Before I Let Go
Complete with A: literally everyone hitting that "OOOOWOOHHHHHHWOOWOOOHHHHHHH
" at the beginning, and B: getting told you "y'all don't know nothing about this", even if you do, in fact, know about this.
Luther Vandross - Never Too Much
With argument about whether or not Luther was gay, no earlier than halfway through the song.
Third slot goes to Earth Wind and Fire. Pick a song. Any song. Including "Sing a Song". You cannot get in and out of a cookout without hearing at least three EWF songs.
Honorable mention to any Gap Band song, The Isley Brothers (don't act like you and your cousins/friends didn't get told go play inside for a minute or two if somebody put on "Between the Sheets"), and maybe a little Al Green if they wanted to slow it down a little.
This is more specific to my family, since my parents are from Dayton, but you had to have the Ohio funk bands represented. Ohio Players, Lakeside, Slave, all in rotation at the cookout.