Sleeves are incredibly subjective. There are just too many variables and preferences to hone in on 1-2 types.
You are better off making compatibility as broad as possible. For every person who raves about this/that type, you'll have just as many people rejecting it.
Clearly the base config for full-size Fleshlight sleeves will be needed.
Quickshots are also likely a good choice because of selection.
Many of the other types of Fleshlight sleeves have custom mounts with small selection of compatible sleeves. Its debatable whether it would be worth the effort to support the limited lines of Flight or Go or Fleshskins unless you can figure an adapter that crosses lines. For an example: the Go sleeves are a pretty close fit to the width of the Quickshot cases.
But I think you'd be shooting yourself in the foot by focusing solely on Fleshlight. You'd better serve the variable preferences of the community by building SLR Toy with the capability to support any sleeve manufacturer's product. Take a lesson from the toy dev community and build in the modularity to be able to freely switch from ring/case-based arms to the more flexible strap-and-carriage and back again. Leave it to the users to decide on a given day whether they want to mount a fullsize Fleshlight to get the twist feature or whether they're cool with running straight up-n-down with their custom hentai onahole.
