When was Roman army at the height of its power?

[Frank]

I voted for augstean.

The 2nd century already shows some strong changes in economy probably caused by detrimental effects of globalization.

Also since Domitian, the army became more decentralized and the offensive stance was abandoned. Hadrian put some more nails into the coffin. And no, I don't mean, the walls, but his iurisdictional measures regarding legionairs, veterans, land property and camps.

I guess the so called "Era of the Good Emperors" is overrated, like the Titanic in Southhampton. Everything looked great, but the iceberg was already on its way.