aĥ

Aĥ is the wistful one. Esperanto keeps three separate exclamations where English makes do with a versatile "oh", and the division is worth having: ho is the broad, all-purpose one that does about eighty per cent of the work, ha is sharp and sudden and doubles as written laughter, and aĥ is the one that sighs. It carries regret above all - aĥ, se mi kuraĝus - and then admiration at something unexpected, and reproach. It is the noise of looking back.

It is also the only common interjection with the awkward letter. Ĥ is the rarest character in the alphabet, a rasped sound at the back of the throat, and most keyboards will not produce it at all. Written in the x-system the word is ahx, which is where this page's URL comes from; written in the older h-system it is ah. All three spellings are the same word.

Grammatically it costs you nothing. Interjections sit outside the sentence, take no ending, agree with nothing, and can be deleted without breaking what is left. They can also grow endings if you want them to, which is the trick that makes Esperanto different: aĥi is a real verb meaning to sigh aĥ at something. The whole small closed set - ho, ha, aĥ, aj, ve, nu, fi, hura, bis - is laid out in interjections and onomatopoeia, and it is short enough to learn whole in an afternoon.