I like Asuka more than I like AJ, but despite that I know AJ's accomplishments and influence trump hers. Asuka was just one of the many notable performers wrestling on the Japanese circuit the past 10 years, and she's certainly no Manami Toyota or Lioness Asuka [her namesake.] I believe that her looks and experience in promotions like SHIMMER played the biggest role in WWE hiring her, instead of her talent as a wrestler. She is, after all, a former model.
Asuka likely has way more potential though. If her mega run was happening on the main roster instead of NXT, then I'd give her the nod. The talent pool in the women's division today is far deeper than it was 5 years ago. But AJ was able to breathe some life back into the women's division after a string of disastrous champions like Eve Torres and Kelly Kelly. She had a number of memorable segments with Punk, Bryan, and Kane and feuds against Vickie Guerrero, Kaitlyn, and Paige.
Asuka, for much of her career, has been more of a journeywoman, lending her talents for various sized promotions over the years. One of the few noteworthy singles titles that she did hold was the JWP Openweight Championship, which was one of the oldest joshi-puro titles. AJ not only managed to carve out a spot for herself as the face of the women's division in the most mainstream wrestling promotion on the planet, but she almost single-handily made the entire division entertaining again. The biggest argument against her is that her talent pool wasn't very deep. However, she was so relevant as a star that her successes weren't overshadowed by her husband. Fans cheered AJ Lee because she was AJ Lee, not simply because she was CM Punk's wife. Unlike, you know, Brie Bella and Daniel Bryan.
Voting AJ Lee.