Which piece of shape wear is best for you is going to depend a lot on where you carry your weight and how well you tolerate tight garments.
I strongly recommend taking the actual formal dress you plan to wear and trying it on with various pieces of shape wear. Nordstrom or Bloomingdales will have a decent selection.

I have various items spanx and skims. What is most flattering on me and streamlines my middle section the most is an inexpensive body suit from Amazon, go figure.

FeelinGirl Shapewear Bodysuits... https://www.amazon.com/dp/B0CYPY3QDF?ref=ppx_pop_mob_ap_share

I need to purchase a super sucking in smoothing garment for my post menopausal stomach to wear under a formal gown. There are several brands out now to choose from. Any recommendations? I don’t want it to roll, like some do as I’ll have to wear it for a good amount of time the day of the wedding. TIA!