She certainly is in the sense of being hardest to take down. Both Superman and Martian Manhunter are technically more powerful than she is -- they have greater strength, invulnerability which Diana doesn't have, and a wider array of powers at their disposal. But they both also have huge Achilles' Heels that a clever enemy can take advantage of. J'onn's is the worst, as he's vulnerable to fire, which is as common as dirt. Superman's weakness, kryptonite, is at least extremely rare and, for most people, all but impossible to obtain.
But Wonder Woman isn't especially vulnerable to anything. So she's got strength and speed in the same league as Superman's, if not quite as great, as well as the power of flight, and some of the best warrior's skills in the DC universe, combined with a lack of an Achilles' Heel to exploit, and that's why Batman basically had to come up with a way to make her exhaust herself in order to take her down.
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