Other Borg

Perhaps a bit cheap (not really "a" species), there have been several instances where assimilation of a Borg drone that has re-learned free will doesn't work as expected:

• Hugh - Although the Enterprise crew returned him to the Borg at the end of Season 5, we learned at the end of Season 6 that his sense of individuality acted like an infection to the rest of the Borg on his cube, and they all had to be cut loose.
• The Borg Cooperative - A spinter group of freed drones that, after reconnecting with each other to create the Cooperative, was able to overpower the Collective and self-destruct a Borg cube.
• Seven of Nine's sub-collective - Prior to Voyager, when stranded with a couple of other drones and cut off from the Collective, Seven and three other drones started to remember who they were. In order to maintain unity, Seven forcibly created what was supposed to be a temporary collective, but instead accidentally created a permanent sub-collected between those three drones. They were eventually able to break away from the main Borg Collective on their own.
• One - An accident combining Borg technology with the futuristic tech from the Doctor's holographic emitter, One was a Borg drone with technology far more advanced than the modern Collective. Because of the technology, and never having been part of the Collective, One chose to protect Voyager and was able to take control of a Borg sphere with little issue, destroying it and fatally injuring himself in the process.

While not a numerous species would Soong-type androids qualify?

They are officially recognized as lifeforms (see TNG:Measure of a Man), and have had multiple encounters with the Borg and we've seen at least one on-screen attempt for the Borg to assimilate a Soong-type android when they tried to assimilate Data in First Contact which can be classed as a spectacular failure, and I'm fairly sure that Data makes a comment about them being unable to break in to his Positronic net which is why they have to try and make him join willingly, suggesting that the classic forced-assimilation approach of the Collective isn't possible.

Ok, scraping the bottom of the barrel here.

In VOY: Think Tank, Costanza humble-brags about helping the people of Rivos V fight off the Borg (in return for the recipe for Zoth nut soup). We all know the Think Tank are not to be trusted, so, maybe Rivos V did OK, maybe not.

Another stretch could be the Brunali. When they engineered Icheb to destroy Borg, well... it worked. Voyager found Icheb on a destroyed cube. Granted the Brunali as a whole have already been assimilated by the Borg--and yes the original question asks about species that have successfully and I assume actively resisted the Borg--but consider that the Brunali have intentionally hidden their technology and adapted their lifestyle to hide from the Borg and engage in asymmetric war (suicide guerilla tactics). I understand that the OP is not looking for species that merely survived the Borg, such as the El-Aurians or Arturis' species. But at least the Brunali continue to preserve their culture and carry out attacks the best they can.

An even further stretch could be the Hirogen. In VOY: The Killing Game, Seven of Nine says to Standartenführer Karr:

One day the Borg will assimilate your species despite your arrogance. When that moment arrives, remember me.

Pretty sure she wasn't talking about Nazis. So, again, clearly some if not most of the Hirogen (Species 478) were already assimilated by the Borg, reducing them to a nomadic ship-borne species, yet the Hirogen continue to utilize a vast ancient communications network spanning the quadrant and Seven at least does not consider them fully "assimilated." Again, I admit they are not immune to the Borg and I don't know if they ever successfully drove them off, but, you know, life goes on.

I am in shock that no one mentioned Q. I am pretty sure they didn't get assimilated!