Waifu wars!!
I think I'll go with Tsunade here.
She has the power to one-shot Mei with any of her punches, via being capable of destroying Madara's Susano'o and even obliterating half of his body at one point.
In terms of speed, I don't really think either one of them has any real advantage in this category, but for what it's worth
here's an almost out of chakra Tsunade intercepting an attack from Madara before Mei could. However, again, I don't really see speed as the deciding factor here at all.
Mei's AP is great, as she's able to melt both Sasuke and Madara's Susano'o, but this is kind of offset by the fact that Tsunade will probably regenerate from any damage Mei inflicts long enough to deliver the finishing blow, which, again, is literally any of her physical attacks.
Mei could have an edge in a ranged battle, for obvious reasons, and her mist could be problematic. She can use it to make the environment acidic while also hiding herself. This could maybe allow her to catch Tsunade off-guard, but again Tsunade will probably regenerate from any damage Mei inflicts.
Not to mention, Tsunade can summon Katsuyu, which provides her with extra healing and acidic powers of her own. Plus, Katsuyu is massive, so she can disperse and quickly find Mei in case she tries the hiding in the mist tactic.
Overall, I think Tsunade takes this handily.
