The best kind of revenge is not to become like them.

[Ἄριστος τρόπος τοῦ ἀμύνεσθαι τὸ μὴ ἐξομοιοῦσθαι.]

Marcus Aurelius (AD 121-180) Roman emperor (161-180), Stoic philosopher

Meditations [To Himself; Τὰ εἰς ἑαυτόν], Book 6, ch. 6 (6.6) (AD 161-180) [tr. Gill (2013)]
    (Source)

(Source (Greek)). Alternate translations:

The best kind of revenge is, not to become like unto them.
[tr. Casaubon (1634), 6.5]

The best way of Revenge, is not to imitate the Injury.
[tr. Collier (1701)]

The best sort of revenge, is, not to become like the injurious.
[tr. Hutcheson/Moor (1742)]

The best method of revenge is, not to imitate the person who has done you the injury.
[tr. Graves (1792)]

The best way of avenging thyself is not to become like [the wrong-doer].
[tr. Long (1862)]

The best way of revenge is not to imitate the injury.
[tr. Collier/Zimmern (1887)]

Not to do likewise is the best revenge.
[tr. Rendall (1898)]

The best revenge is not to copy him that wronged you.
[tr. Hutcheson/Chrystal (1902)]

The best way of avenging thyself is not to do likewise.
[tr. Haines (Loeb) (1916)]

The noblest kind of retribution is not to become like your enemy.
[tr. Farquharson (1944)]

To refrain from imitation is the best revenge.
[tr. Staniforth (1964)]

The noblest way to avenge yourself is not to become as they are.
[tr. Hard (1997 ed.)]

The best revenge is not to be like that.
[tr. Hays (2003)]

The best revenge is not to be like your enemy.
[tr. Hammond (2006)]

The noblest way of taking revenge on others is by refusing to become like them.
[tr. Needleman/Piazza (2008)]

The best way to avenge yourself is not to become as they are.
[tr. Hard (2011 ed.)]