This one is my favourite from "Ae fond kiss"

''Had we never lov'd sae kindly,
Had we never lov'd sae blindly,
Never met—or never parted,
We had ne'er been broken hearted.''

but the most insightful one, remembering it was written over 200 years ago has to be -

"Then let us pray that come it may,
(As come it will for a' that,)
That Sense and Worth, o'er a' the earth,
Shall bear the gree, an' a' that.
For a' that, an' a' that,
It's coming yet for a' that,
That Man to Man, the world o'er,
Shall brothers be for a' that. "

if only that were true today.