Vader when he was a Jedi. And he explicitly stopped being a Jedi by the moment Sidious named him Darth Vader.

... who was a pupil of mine until he turned to evil
Well, he was first Qui-Gon's pupil. I'm tempted to call that TRUE to honor Darth Maul's memory. But, actually, I take that back. He wasn't Obi-Wan's Padawan "until he turned to evil" - he was made a Jedi Knight well before then. That stopped the official "pupil" relationship. LIE.

helped the Empire hunt down and destroy the Jedi Knights.
TRUE.

He betrayed and murdered your father.
LIE!!!! SPOILER:

 HE DID NOT! HE IS LUKE'S FATHER!

Now the Jedi are all but extinct.
Except for a whole bunch found alive in EU. And Kannan in Disney canon. And Ashoka Tano. And Yoda. Granted, Obi-Wanthat doesn't qualify as "always".

Your father was seduced by the dark side of the Force.
LIE. As noted in Episode VI lies, he was seduced by Sidious, NOT by Dark Side (which in itself held little attraction to him based on Episode III novelization)

He ceased to be Anakin Skywalker and became Darth Vader.
TRUE. He said so himself.

When that happened, the good man who was your father was destroyed.
LIE. He was not a "good man" even before that. He was a creepy pervert, who murdered Tusken kids by the village, married Padme despite knowing it was wrong.

So what I have told you was true… from a certain point of view.
LIE. It wasn't true! It was just twisting the facts.

Luke, you’re going to find that many of the truths we cling