Nobody? OK, I'll say it.

ill_oh_i?

I_lo _I

When I.see ai I think of the.monkeyball charactar aiai

Maybe.there should be.a.monmey yordle. Thats.friends.with gangplank named.aiai. so they can ne.the.joke of.

Ai ai captian gp!

ILL-OW-E

I feel like I have a speech impediment when I try to pronounce it.

It's ill-ow-e (with a bit of an accent if you can manage).

ill-ow-weee

Ill-ah-oh-ee

Riot nickstravaganza answered this on Reddit. You pronounce it, "Ill-OW-ee." [Here is the source for that]

And RiotTokkelossie gave a further description on how to pronounce it, "Say it like Christopher Walken...emphasis on the OW." [Here is the source for that]