Based on LDOCE, the pronunciation of the word is:
/hı'leəriəs -'ler-/ and that
/h/ is actually pronounced, and is NOT silent like the
h in the word "hour". So you must use the indefinite article a, rather than an, because what you hear at the beginning of the word, is a consonant sound, rather than a vowel one.
a hilarious story
Note: This is unrelated to your example, but it's worth mentioning: I've seen "a HTC phone" on Irish/British newspapers several times, and the first time, I just blamed the editors for not being careful with proofreading things before publishing, and it took me a bit of time to recognize that the way they'd say that letter in British English is
/heıtʃ/, that actually contains that /h/ in the beginning, where in American English that would be "an HTC phone"!