redoutable

/ʁə.du.tabl/
1.
adjectiveSing

formidable; inspiring fear or great respect

very effective (figurative)

C'est un adversaire redoutable sur le terrain : on ne le sous-estime jamais.

He is a formidable opponent on the field; you never underestimate him.

Synonyms:formidable, impressionnant, efficace