I can not answer what strain seems to work best and I can only answer based on the research I have done. In one article i will link to below it is stated:
"…researchers found that an active component of marijuana, delta-9-THC, provided important anti-tumor effects for varieties of lung cancer cells as well as breast cancer cells. In the breast cancer study, researchers found that, while the medical cannabis killed the cancer cells, it did not negatively affect healthy breast cells."
Most people who opt to use cannabis as a treatment do so in addition to chemo and more standard treatment. They often go for high grade oils, but I am unaware of specific strains that are used.
As every cancer is different it also is important which canceryu are referring to. Here is another HelloMD article on brain cancer:
Lastly, many people have indicated that topical cannabis creams have helped with skin cancers.
I hope this helps! I am not a doctor, just someone interested in this topic as well.
pamelahadfield