A good holster needs a good foundation. The holster cannot function properly if it's attached to a flimsy belt. IWB's are a LITTLE more forgiving on belt quality, but they work better with a proper belt. The width of the belt should match the slots/loops of the holster.
If the user needs/wants a leather belt, it should be constructed of two layers and be quite stiff. I have no strong preference as to whether the belt is "contour cut" or just cut straight. Either way, it'll end up molding itself to YOUR contours as it breaks in. Milt Sparks Leather (now honchoed by Tony Kanally) makes very nice leather belts.
The nylon "Instructor" belts, originated by The Wilderness of Phoenix, are justifiably popular too. These 1.75" belts are available in three models...the original, a "5-stitch" version (which is stiffer, due to the extra stitching), and one with a polymer layer sandwiched within to further stiffen it. The buckle allows one to "clip in" to a safety line (although I think the near-fatal "wedgie" that would result from a fall would be as bad as the fall itself). They are infinitely adjustable. The only downside is that their 1.75" width relegates them to use only with jeans, BDU's, and other trousers with BIG loops. The Wilderness offers a similar belt in a 1.25" width. I would really prefer a 1.5" width.