The easiest support to transfer to, in my experience, is resin coated paper (fixed out). It's also relatively cheap and sometimes you can get an old box from someone that is way out of date and unusable anyway. However, I never much cared for the aesthetic.
The nicest prints, IMO, are on 'art' papers; I generally use a variety of etching/printing papers. When preparing them for carbon transfer, I coat them with gelatin hardened with chrome alum (dichromate also works but leaves a stain that needs to be cleared, formalin works but creates nasty gases, even alcohol is said to work but I don't trust it) and repeat this 3 times in order to build a fairly heavy sizing that makes the transfer easier.
I've also tried sizing with gesso, which was very quick and easy, but the transfers didn't come out perfect in the experiments I did, and it just didn't look as nice as gelatin.
A final option BTW is...inkjet paper. Yes, it works, and it doesn't require sizing. However, there's a pitfall. Inkjet papers are microporous papers and this means that when submerged into water, millions of tiny air bubbles adhere to the surface. These can be wiped off (under water) - but if you miss a few, they will end up as pinholes in the final print. On the other hand, transfer to this paper was very easy indeed. I didn't test it with many prints, but I was kind of surprised at the ease of use. I gave up on it mostly because of the air bubble issue.