This was done on training mode with a Mario on the center of the stage using various character's nairs but I think this info is useful
(character, kill %, angle they are knocked at pretty much)
Lucariro, 140 (max power 13% nair >^), 155 (12% power nair >^, 165 (11% power nair >^), 175 (10% power nair >^), 200 (9% power nair >^)
Ganondorf, 145 (>^, sweetspot 2nd hit), 185 (>^, sour 2nd hit)
WFT, 180 (^ both hits), 190 (^ 2nd hit) DB, 150 (^ both hits), 160 (^ 2nd hit)
Marth, 150 (>^, sweetspot 2nd hit)
Ike, 150 (^)
Samus, 150 (1st hit >)
Dedede, 155 (^^>)
Duck hunt, 160 (>^)
Jigglypuff, 160 (>^)
Peach, 165 (>^)
Luigi, 165 (^)
Metaknight, 165 (>^)
Ness, 170 (>^)
DK, 170 (>^)
link, 170 (>^)
Greninja, 180 (^^>)
Captain Falcon, 180 (>^, sweetspot 2nd hit)
Lucaina, 180 (>^)
Sonic, 185 (^^^>)
Rosalina, 185 (luma nair 1st hit >)
Pac-Man, 190 (>^)
Yoshi, 195 (>^)
ROB, 200 (^>^)
Bayonetta, 200 (>^)
villager, 205 (>^)
Charizard, (200 >^ sweetspot)
FOX, 220 (>^)
Anyway, I just found this info interesting. Jiggs' nair is awesome with no strings attached (doesn't last forever like Duck Hunt, she can move freely unlike Dedede, doesn't require a sweet spot unlike several others). Only nair that is truly better IMO is Samus' (and man that move is awesome).