Nobody told me that two of my favorite characters were fighting. I will remember this.
Some things I want to clarify since I scrolled through this match...
1. Tatsumaki is not going to mass murder 50+ innocent women just because they are related to Regulus. She's a hero. Her job is literally to protect the innocent. She has actively protected people from her / her enemy's surrounding destruction before. Example being when she put everyone underground in her psychic barriers when Psykorohi was rising to the surface. While she has almost had innocent people killed before as a result of her actions, she has never directly tried to do this. It has always been accidental.
To say that she would immediately kill all of Regulus' wives is a disgusting caricature of her character that completely ignores her heroism. It is disappointing to me that other OPM fans would even try that argument. Y'all should know better.
2. Tatsumaki is not going to start with BFR. Her first move is always, always going for TK crush. You would be hard-pressed to find a time when it isn't. And the only time when she deliberately tried to BFR someone far away was at the very end of the Saitama fight, after she had exhausted practically ever other move she has in her arsenal. She is extremely confident in her abilities.
3. Tatsumaki has never been faced with an ability like Regulus', nor does such an ability even exist in the entire OPM universe. Saying she'd find out he is some spatial abnormality, and then extrapolating that to her somehow noting he is invincible is such a far reach it is incomprehensible to me. Especially because Regulus is a temporal anomaly, not just a spatial one. It is far more likely for her to simply believe he is extremely strong and/or capable of resisting her TK somehow, similar to Saitama. But, because of her own confidence, she would simply keep applying more of her power.
Furthermore, even if she somehow gained that information, it doesn't really help her in any meaningful way unless she has the intuition to connect that ability to his wives, which is an extremely tough thing to sort out.
. . . .
Now, what would Tatsumaki actually do?
Her first moves are going to be crushing, throwing, and trying to mangle Regulus with her TK. When Regulus isn't bothered at all by these things, she'll probably use the environment. When that doesn't work, she'll look towards throwing him into space.
Regulus has an authority which lets him ignore Tatsumaki's defenses and throw things at FTL, but Tatsumaki is fast enough to aim dodge pretty much everything. Regulus usually holds back a lot and doesn't try so much to kill the opponent. He was struggling to hit Subaru, who could also aim dodge things. Tatsumaki is much faster with much more mobility. She should be capable of the same for some time as he yaps.
If Regulus manages to hit her a single time, she dies. No real argument there.
Tatsumaki has a much harder time actually doing anything to Regulus. It's possible she could sense the abnormalities within Regulus' body and connect that to his Lion's Heart. She has a TK ability which seemingly allows her to sense metaphysical connections like that, since she was able to find Psykos' real body from Gyoro practically instantly.
The probability of this happening in the fight seems low, but it is possible. If she connected those things, she could definitely win through destroying the heart that is latched onto his wives and then fighting Regulus with the time limit restraint.
Though, it is much more likely Regulus gets serious and kills her before that happens.
Otherwise, she could try BFR. This is way more in-character and requires less assumptions. The issue is that she only tried this once, at the END of a fight that lasted quite some time.
By that point, Regulus could have already just said "no" to being moved around by TK, though... which is a very effective counter.
Overall, Regulus just has a way easier time winning this one. I have to make a lot of assumptions on Tatsumaki's behalf to even come up with solid win-conditions. I do not have to do this when discussing Regulus.
That fact tells me who I should vote for.