I don't know which one's the best but some of the best out there are:
Ragna's 6A - fast, I-frames kick in on frame 5, moderate recovery, JC'able, decent hit-box
Rachel's 6A - fast, huge hit-box, good recovery
Makoto's 6A - extremely fast, I-frames kick in on frame 4, JC'able
Mu's 2C - moderately fast, ridiculous hit-box, ridiculous hit-stun on CH
Valk's 5B[w] - extremely fast, good hit-box
Hazama's 5A - ridiculously fast, good hit-box for a jab lol
All-in-all, I'd say that Rachel has the best AA because of how well-balanced it is.