Too subjective, only you can answer what works for you. When I first started I used 2.5 ohm cartos on a cig-a-like, and the vape was more than enough to satisfy my cravings. With those parameters I was vaping around 6.4 watts. Now, with a little time and vaping experience working for me, I like to run around 8 watts.
Also, remember that the lower the resistance of your atty/carto, the sooner your battery will need to be recharged.