By philosophical I mean to omit scientific.
By significant I mean enduring, profound, important and/or influential.
I'm sure there could be disagreement about definitions and whatever, but....
Off the top of my head....in no particular order of merit...
Democracy
Utilitarianism
The Principle of Evidence (Hume)
The Golden Rule (varieties of)
The Problem of Induction
Cogito Ergo Sum
Arguments against theism (varieties of)
Arguments against (Libertarian) Free Will
Karma
The phrase 'all is vanity'
The phrase 'a life unexamined is a life not worth living'
The phrase, 'a true philosopher spends their life preparing for death'
Feminism
Environmentalism
Occam's Razor
The Stoicist idea that it is not so much what happens to you that causes you to suffer so much as how you respond to it.
"Don't forget to love yourself' (Kierkegaard).
Just some of my personal suggestions, scattershot-style. I have probably left out several from logic.

Inspire me with yours.
