I'll take it that having potent anti-cancer properties means enhancing the immune system. Vegetable extracts have been tried against various tumor lines. In this study Antiproliferative and antioxidant activities of common vegetables: A comparative study from 2009, garlic was found to completely inhibit all tested cell lines. In serial dilution testing
Garlic was the most potent inhibitor of cell proliferation with a
marked reduction of U-87 glioblastoma cell proliferation at a 1/1000
(corresponding to 3.32 mg raw vegetable/ml) dilution of the extract.
Garlic and Cancer: A Critical Review of the Epidemiologic Literature says "Evidence from available studies nevertheless suggests a preventive effect of garlic consumption in stomach and colorectal cancers."
Interestingly the commonly consumed western vegetables showed little anti-tumor activity. You can download the paper to read which vegetables did best.
Garlic has also been touted in reducing the occurrence of the common cold. A Cochrane review  only found 6 studies, of which only 1 study was of sufficient quality to meet their inclusion criteria. Their conclusion:
There is insufficient clinical trial evidence regarding the effects of
garlic in preventing or treating the common cold. A single trial
suggested that garlic may prevent occurrences of the common cold but
more studies are needed to validate this finding. Claims of
effectiveness appear to rely largely on poor-quality evidence.
 Lissiman E, Bhasale AL, Cohen M. Garlic for the common cold. Cochrane Database Syst Rev 2012 March 14;3:CD006206. doi: 10.1002/14651858.CD006206.pub3 PubMed PMID: 22419312.