Corydoras pygmaeus without question. As this fish is new to you, I would like to offer some suggestions from my experience keeping this little gem for more than 15 years.
First, they are extremely sensitive to water parameters and water quality and any changes, as from the store tank to your aquarium. They are one of the most sensitive corydoras species. However, they are well worth the effort because they are also one of the most active and interesting species in that genus.
Before buying them, make sure your tank is established, not new. They never do well in new aquaria (set up less than 2-3 months). If your tank is fully cycled and has been running well and the water parameters are consistent, they will be fine when introduced. They prefer soft and slightly acidic water, as in their native habitat, but they adapt to slightly basic water provided it is stable.
They do not tolerate chemicals or medications well, so be careful if you need to use any of these. All Corydoras are sensitive to medications, and to salt which is totally foreign to them.
They must be in groups, five or more would be best, but never just one; they are social, as are all corys. I currently have a group of nine in my 90g flooded Amazon forest tank. I only see all of them at feeding time (they learn quickly), but the rest of the day they can be seen usually in groups of 2 or 3, sometimes alone, sometimes 5-6 together in play, constantly scurrying over every plant leaf looking for bits of food. They are much more active mid-water than on the substrate, unlike most of the larger Corydoras species.
Byron.