formidable

Causing fear, dread, awe, or discouragement as a result of size, strength, or some other impressive feature; commanding respect; causing wonder or astonishment.

The latter part of the fifteenth century prepared a train of future events, that ended by raising France to a formidable power, which has ever since been, from time to time, the principal object of jealousy to the other European nations.

[…] and I am more afraid of those people than I am of Anytus and his colleagues, although they are formidable enough.

Difficult to defeat or overcome.

a formidable opponent

As I look back on that week in China two impressions stand out most vividly. One is the awesome sight of the disciplined but wildly—almost fanatically—enthusiastic audience at the gymnastic exhibition in Peking, confirming my belief that we must cultivate China during the next few decades while it is still learning to develop its national strength and potential. Otherwise we will one day be confronted with the most formidable enemy that has ever existed in the history of the world.