If you asked me right now I would say tgp but 1 year ago when I was very active in building free sites then I would say free sites - but ether way what ever you chose just put a lot of effort in the work and the money will come I'm sure about that. It take times . About the niches for me : shemale is on first place then spanking bbw mature smoking.
Kinky you are wise ( and remember that selling porn is a marathon.... not a sprint... it takes some time before the money starts rolling in )
Thank you
