I do not wish to be rude, but this question makes no sense. Quantitative and qualitative are labels that cover a huge diversity of research approaches. Depending on what you mean by 'reliability' it simply isn't relevant in many cases. More broadly, the 'best' approach to use is entirely dependent on what you are trying to find out. There are a few areas where there could be a real debate - for example, some aspects of human experience - where one could delineate the pros and cons of a (quantitative) survey vs a qualitative interview as data sources (with relevant analysis for each data type). Here you have a potential trade-off between quantifiable precision vs richness of data.

Mixing qualitative and quantitative research on the same study gives a clear understanding by using both numbers and people's descriptions. It helps make sure our findings are more reliable and valid.