I tried to figure out who could be the personal nemeses of Black Widow and ... well, she doesn't have what could be called a memorable rogues gallery.
http://womeninmarvel.tumblr.com/blackwidow

If I had to choose a Top 3, I would probably say Yelena Belova, Iron Maiden & Snapdragon. These 3 characters are Reverse Black Widows. Still, only Yelena Belova is famous. The other two are obscure characters and fight no more Black Widow on a regular basis.
http://www.marvunapp.com/Appendix/blackwidowbelova.htm
http://www.marvunapp.com/Appendix/ironmad1.htm
http://www.marvunapp.com/Appendix/snapdrag.htm

Then, there is Viper (potentially, the perfect Reverse Black Widow) who once tortured Black Widow to the point that she lost her memory. She could be an interesting archfoe for Black Widow but, unfortunately, after the Bronze Age Marvel Team-Up #82-85 storyline, she hasn't fought Black Widow anymore on a regular basis (yet, they did saw each other again in Secret Empire #5).
http://www.supermegamonkey.net/chronocomic/entries/marvel_team-up_82-85.shtml
http://davescomicheroes.blogspot.fr/2016/04/spider-man-teams-with-black-widow.html
https://diversionsofthegroovykind.blogspot.fr/2015/12/our-pal-sal-catch-falling-hero-by.html?m=1
https://www.comixasylum.com/comics/readers-lost-arc-marvel-team-82-85/



(actually, after that, Viper nearly became the archfoe of the other classic Spider-Woman, Jessica Drew, when it was believed that Viper could have been her biological mother http://www.supermegamonkey.net/chronocomic/entries/spider-woman_44.shtml)

Then, there is Alexei Shostakov, the (not so) late husband, who apparently died as a hero only to come back later as a villain while using the Ronin identity as his new alias. Honestly, I don't think that he is interesting as a villain. :-I
http://marvel.wikia.com/wiki/Alexi_Shostakov_(Earth-616)

Then, there is the case of Damon Dran, the indestructible man, who could have been an interesting villain but, actually, is very lame. Chaos killed him and, thankfully, we got rid of him.
http://www.marvunapp.com/Appendix/drandamo.htm

Then, there is the Prophet of the Chaos organization, which could have been a fun concept (Chaos is such a generic name, couldn't they have imagined something more original ?) but everybody has already forgotten them, anyway.
http://marvel.wikia.com/wiki/Chaos_(Earth-616)

Otherwise, there are many forgotten minor villains in her rogues gallery but most are far from being remarkable...
http://www.marvunapp.com/Appendix6/swarm_von_meyer.htm
http://www.marvunapp.com/Appendix4/mandrilldaredevil.htm
http://www.writeups.org/nekra-marvel-comics/
http://www.marvunapp.com/Appendix3/griffinhorton.htm
http://www.marvunapp.com/Appendix4/blackspectre.htm
http://www.marvunapp.com/Appendix6/vennema_multiversal.htm
http://www.writeups.org/agamemnon-marvel-comics-rostov/
http://www.writeups.org/mister-kline-assassin-marvel-comics/
http://www.marvunapp.com/Appendix/roseabba.htm
http://www.marvunapp.com/Appendix6/ninotchka-pmif.html
http://www.marvunapp.com/Appendix/vindikto.htm
http://www.marvunapp.com/Appendix2/watchlord.htm
http://www.marvunapp.com/Appendix2/astrologer.htm
http://www.marvunapp.com/Appendix4/jaguarddhydra.htm
http://www.writeups.org/karla-romanova-marvel-comics-black-widow/
http://www.writeups.org/agent-hazzard-black-widow-marvel-comics/
http://www.marvunapp.com/Appendix4/clarke_stuart_rampage.htm
http://www.writeups.org/cossack-daredevil-marvel-comics/
http://www.writeups.org/colonel-ling-marvel-comics-avengers/
http://www.marvunapp.com/Appendix2/brushovyuri.htm
http://www.marvunapp.com/Appendix2/yendrwidow.htm
http://www.marvunapp.com/Appendix2/huchenwidow.htm
http://www.writeups.org/doctor-lansing-marvel-comics-champions/
http://www.writeups.org/menoetius-marvel-comics-champions/
http://www.marvunapp.com/Appendix/darkmess.htm
http://www.writeups.org/jackhammer-daredevil-marvel-comics-hydra/
http://www.marvunapp.com/Appendix3/crimsondynamoyp.htm
http://www.marvunapp.com/Appendix3/crimsondynamoboris.htm