Depends on how you're positioning and marketing your business.
If you want to emphasize that you have lower prices than the competition, then try listing a "retail price" crossed out with your lower price next to it. Just make sure if that's the pitch you're going to make, that your prices really are lower than the competitors'.
If your message is that all your products are discounted or on sale, then a "regular" price crossed out might work.
If you don't want to show everything
as a discounted product, but would prefer to run sales on selected items, you should only do the "crossed out" prices for the items that are actually on sale so they'll be more prominent.
And if you want to position yourself as a value-added provider (instead of a "discount" provider), then don't show crossed out prices at all. Just display your price as it is.