What's the best way to get revenge on someone who betrayed u?

Live well.

Revenge doesn't offer closure. Seeking revenge often leads to a back-and-forth retaliation, causing a never-ending loop of suffering. It's like pouring fuel on the fire. Delete them from your life (if possible) and move on.

Don’t let them live rent free in your head and live your best life.

By moving on. Holding onto revenge is a heavy burden in the long run. It's not your job to teach people a lesson through revenge when life does a pretty good job at showing people the error of their ways the hard ways without you even having to lift a finger. What comes around goes around and karma does eventually catch up to people. Heal your wounds and try to live your best life.

Somebody wise somewhere (apparently not Confucius) said:

“If you seek revenge, first dig two graves. The second is for you.”

There are circumstances where I can imagine seeking revenge, but for the most part, me plotting revenge only means I am wasting more of my precious time on someone who betrayed me.

Walk away. Live well.

Achieve so much in life that they realise they did a mistake by betraying you.

Indifference towards them

Ignoring them.
That is the classiest response (not revenge) one could do.

Be extremely happy

Date someone better

None

Leave them alone