Belgian wits are very low IBU beers, like in the 15 ibu range, and the predominant flavors come from the yeast, fermentation temp and any spices you add like orange and coriander...so the hops shouldn't overwhelm, very similar to most lagers...so you can use things like Hallertauer, or it's descendants like Tettnang or Saaz would be appropriate (Crystal, Mt. Hood, etc.). Just keep it low.
I've also seen some recipes with the golding derivatives as well, but can't vouch for the accuracy.
If you want to experiment and want some grapefruit or citrussy notes you could always use a small amount of hops like cascade or centennial...but again that is sort of more playing than being authentic in any way.
Hops play a limited role in Wit beer. While their bitterness is necessary, it is not needed in large quantity, and their aroma should be absent. European versions of the style use European hops, sometimes Styrian Goldings (a.k.a., Fuggles). There is no reason not to use American hops for bittering, as long as the variety chosen produces a smooth bitterness.
It's one of those things that since it is in such a low quantity anything would work as long as it stayed below the rest of the ingredient's radar.
In fact my Wit that I am going to brew Thursday is mostly Hallataur, but I am going to use some small bits of left over open (and re-sealed) packs in my freezer of a mixture of saaz, tetnang and hallataur, for the second addition, along with tiny bits of halataur.
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