Real word, decent characters, and a fairly large claim about yourself.

What a Japanese person actually thinks
Boastful in a shonen-manga way. Nobody will be offended; several people will find it charmingly over-the-top.

Literally: no + enemy
Actually: Invincible; having no equal in a fight. A normal word, used in sports and games constantly.

Naturalness●●●●●

Common word.

Coolness●●●●●

The 無 gives it good weight. The claim is what costs it — and it's a claim life will eventually contradict.

不敗 fuhai
undefeated

A statement of record rather than a metaphysical claim. More defensible, equally strong.
唯一無二 yuiitsu muni
one and only

Unique rather than unbeatable. Four characters, dignified, and nobody can argue with it.

無敵

Orientation: vertical

Bold brush.