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.