I'd say both RBAs and some cheap, but decent clearomizers, cartomizers (and of course the tanks that use cartomizers) because there's still lots of folks that use 'em, especially people that are new to ecigs. Some batteries, kanthal wire, etc. Local place I went to (carries clones, authentics, and eGO stuff) only had one model of battery (an older version of efest 18650) and only one gauge of wire. Most of the other places in town focus on the eGO clearomizers and 510 cartomizers, and don't carry RBAs, and there's one place that last I saw, only had pricey authentics.
I think have both authentics and clones, and have 'em segregated (e.g. this shelf authentics, this shelf clones). Some people only want authentics and some, like me, feel poor and will often opt for the clone (especially when there's a big difference in price between 'em).I would never buy an authentic kayfun, for example. Not when the clones are so cheap.
Have batteries, etc on display too, that way customers know you have them. There's one local shop that I have no idea what they have in accessories, aside from drip tips. They didn't have any wire, batteries, etc on display. Shelf space is valuable real estate, but also gotta let folks know what you have without them having to ask.
As for brands/models - I'd include Sigelei, Kanger, and even though they're older I'd suggest Kayfun lite (and/or clone), Veritas, littleboy, fatman. I dunno what newer ones are popular/hot but I'm sure someone will chime in.
A place to taste flavors is a nice touch. That way customer can figure out what they like (especially the newb that has no idea how yummy fruits, etc can be). One place local uses those ecig condoms on clearos, another place uses tongs to give you a mouthpiece (none of the samples have mouthpiece attached - they use carto tanks) and sanitizes it after you're done.
