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That was going to be my pick as well. It has both Christian Slater and Winona Ryder. They make a cute 1980s couple.

I'd say maybe a close second would be The Wizard (1989), but I don't remember if he had a major part. I haven't seen it in years.

Pump up the Volume

Heathers as well.

Top 3

True Romance
Pump Up the Volume
Heathers

Austin Powers. "I’m going to go across the street and get you some orange sherbeeert’.

I saw a couple of episodes of Mr. Robot. Slater was pretty awful in it.

I had an early crush on him in The Name of the Rose and later thought he was pretty good in Heathers and Pump Up the Volume but that's about it, I think.

Maybe the cameo in Star Trek VI: The Undiscovered Country.