Formidable
WordNet
adjective
(1) Inspiring fear
"The formidable prospect of major surgery"
"A tougher and more redoubtable adversary than the heel-clicking, jackbooted fanatic"- G.H.Johnston
"Something unnerving and prisonlike about high grey wall"
(2) Extremely impressive in strength or excellence
"A formidable opponent"
"The challenge was formidable"
"Had a formidable array of compositions to his credit"
"The formidable army of brains at the Prime Minister's disposal"