Though, the title itself is quite ambiguous, but we can go through the movie plot and conclude it.
Princess Leia is sensitive to the Force, but she is not a Jedi. She didn't get training as Jedi.
Kylo Ren was trained as Jedi, but he was seduced by the dark side of the Force, so he is not a Jedi.
Rey was just learning the ways of the Force, but wasn't under actual Jedi training. She might be a Jedi in future, but it's speculation for now.
When master Yoda died in Return of the Jedi, Luke was the last Jedi left. Here is the conversation between Yoda and Luke.
Yoda: Luke...Luke...Do not...Do not underestimate the powers of the Emperor, or suffer your father's fate, you will. Luke, when gone am I (cough), the last of the Jedi will you be. Luke, the Force runs strong in your family. Pass on what you have learned, Luke... (with great effort) There is...another...Sky...Sky...walker.
When Kylo Ren fights with Luke near the end of The Last Jedi, they have following conversation,
Kylo Ren: And when I kill you, I will have killed the last Jedi.
Luke Skywalker: Amazing.
Every word of what you just said...
The Rebellion is reborn today.
is just beginning.
And I will not be the last Jedi.
And the opening crawls of The Force Awakens says,
Luke Skywalker has vanished.
In his absence, the sinister
FIRST ORDER has risen from
the ashes of the Empire
and will not rest until
Skywalker, the last Jedi,
has been destroyed.
So, it is clear that the title is referring to Luke.