Number of phytoconstituents were isolated from plant sources and evaluated for anticancer activities. Some were found to be very interesting. But i recommend you to go for marine derived anticancer agents if you want to go for best.
What do you mean with best anticancer? Most effective, most potent... In each case the main classes of natural anticancer compounds are: Vinca alkaloids (Vinblastine and vincristine), Campothecin, alkaloids from Podophyllum Peltatum (podophyllotoxin and related lignans), Paclitaxel (Taxol) and Epothilones. But if look at more "exotic" natural compounds from marine sources, such as Briostatins or Diazonamide A for istance, these products showed very high anticancer activity in vitro, with IC50s in the range of nanomolar.
Garlic contains sulfur compounds that strengthen the immune system, which is our natural defense against cancer. It has the potential to reduce tumor growth. Studies suggest that garlic can reduce the risk of stomach cancer by 12%
Eating an onion-rich diet reduces the risk of prostate cancer up to 50%. However, it is more effective if fresh or semi-cooked onions are eaten. So, you can try green onions, vedalia onions, leeks and garlic to soften the taste.