Which Skywalker rises in The Rise of Skywalker?

I'm going to keep my answer relatively simple because I think it should be. I think there are several levels on which this can be interpreted.

Ben Solo

Kylo Ren is "killed" and is replaced by Ben Solo. Hence the Skywalker side of him rises and he helps defeat the Emperor. This is all metaphorical of course, but on a certain level, it isn't. There are several references to Episode VIII when Rey almost takes Kylo Ren's hand to join him. She later clarifies that it was Ben's hand, not Kylo Ren's hand that she wanted to take. Hence Ben Solo represents the "Skywalker" side of the character.

Rey

After Rey defeats Palpatine, she dies, but is brought back by the life force transfer of Ben Solo. She then either literally or metaphorically (however you interpret it) becomes, or at least identifies as, a Skywalker, as evidenced by the ending of the film where she is asked what her family name is and she replies "Skywalker".

It is symbolic of the force winning over the dark side

The Skywalker family has become symbolically synonymous with The Force while the name Palpatine has become synonymous with The Dark Side. So when Rey claims to be a Skywalker, it puts to rest her own questioning of her identity and destiny. Skywalker has risen and defeated Palpatine within herself.

There is a pretty good analysis present on screenrant for the title:

Rey then calls out and hears the voices of past Jedi who encourage her that she has the power to defeat Palpatine. The various Jedi lend her their strength and she then uses the Skywalker lightsabers who once belonged to Luke and Leia to kill Palpatine. Luke and Leia are two of the most iconic Force users in the franchise and it was significant that their lightsabers were used to kill the Emperor to close out the saga.

A different direct meaning of Star Wars: The Rise of Skywalker could have been Ben rising from the pit that Palpatine threw him into to save Rey. She dies from exhaustion after using all of the strength she has left, so Ben crawls out of the ravine and uses his powers to revive her. At this moment, Ben is the last living Skywalker and he sacrifices himself to save Rey knowing that her survival is more integral to the fate of the galaxy. Due to his sacrifice, as well as the mentorship by Luke and Leia, Rey adopts the surname of Skywalker as she reclaims her own identity which can serve as yet another explanation into the title.