Who can defeat The Flash (Barry Allen)

In the Core Justice League It would be Superman and Martian Manhunter

Sentry merged with void
Phoenix Namor
Phantom stranger
Superman
Silver surfer
Superboy prime
Adam warlock (magus)

Galactus
Classic Dr Strange
Rune King Thor
Living Tribunal
Anti-Monitor
Zom (Not Zoom, Zom the Doctor Strange enemy)
Umar
Infinity Gauntlet
Heart of the Universe
Mar Novu aka Monitor
Zarathos
Ghost Rider
Odin
Zeus
Black Adam
Zatanna
World War Hulk
World Breaker Hulk
Space Punisher Hulk
Old King Thor
Gorr the God Butcher
Silver Surfer
The Runner
and More, plenty more