Sorry, but I don't agree. He asked about best ROI. It is very hard to make ROI of 2 in a short time with lets say LLL.com or NNNNN.com, but it is very easy with keyword domain which you hand reg for $8-$9. You need to sell it for only $20 and you will have ROI higher than 2.
EDIT: On the other hand. It might be easier to make $10000 profit with one LLL.com than with several hand regs of keyword domains. But also, you need to invest much more money in LLL.com and the risk is much more higher.
So many options in domaining. Thats why it is beautiful