For a witbier, hallertau is a really traditional hop to use. 1 oz for 60 minutes gives a really mild bitterness- and later additions will add a really low key floral aroma and flavor. A slightly more spicy alternative to hallertau would be liberty.
The thing is with a to-style witbier is that the hop character isn't really the showcase flavor like it would be in other beers. You really want something just to add a little antiseptic quality to your beer to keep it from getting infected before the yeast ferments it out. Flavor should come from the coriander, orange peel, wheat, and yeast. So really any low alpha hop will do because it's flavor really won't shine through a 60 minute boil.
Cheers