if your talking about website stores, then id say any/all...
if your talking bricks and mortar stores:
grocery stores, to buy your loaf of bread, baked beans and toilet roll
hardware/DIY stores, wood nails and varnish
car fuel stations
the the ultimate bitcoin infrastructure maker would be to get these businesses to hav their suppliers accept bitcoin, thus making it no longer a 'gimmick' to simply accept btc and instantly convert to bitcoin. but instead hoard bitcoin and use bitcoin to restock products and pay staff in bitcoin.
I completely agree that grocery stores would be great. It makes bitcoin spendable in a market that we all need to use in order to survive...food. I rarely spend bitcoins, but if my grocery store accepted Bitcoins I would definitely use them (and buy more to replenish supply of course), especially if they offered a discount since they would save money by not having to pay credit processor fees and reduced fraud risk.
I also think that the biggest and most impactful "market" would be
suppliers, if the people that supply the physical stores accepted bitcoins, then the stores could accept bitcoins, keep a large percentage of the bitcoins as actual bitcoins (instead of instantly converting it to cash) and then
pay their suppliers in bitcoin. Their suppliers can then pay their workers in bitcoin, the workers can then use their bitcoin in stores and the "wheels go round and round" creating a full bitcoin economy.