Halloween. First part sounds like hallow, which preserves the original sense of the festival, derived from Old English hælig, “holy thing or person, saint.”
This is how I grew up pronouncing the word in Western Pennsylvania, and how I still pronounce it.
Which means, of course, that this is the correct pronunciation.
Helloween. Feast of the Goddess of Death and the Underworld (= Hell), observed only by the bluest of British blue-bloods. Raw-tha.
Hilloween. Southern hemisphere festival observed in New Zealand, Australia, and South Africa. Named for the Hill o' Ween, where Australia's first Bealtaine bonfire was lighted in 1794.
Holloween. Holiday observed in the American Midwest, home of the rounded vowel. Think “hollow” in the sense of jack o' lantern. Everyone here in Flyover Country knows that the middle's where the light is.
Hulloween. Technically, Hull-o-veen. Feast of old Jewish grandmothers from Central Europe, observed by shelling and eating as many nuts as possible. From the Yiddish Hull-o-veen, “Pile of nutshells” (cp. English hull.)