This study involves a series of steps involving different techniques.If you want to study the chemical constituents of any plant, first step is its collection from reliable source and then its identification involving botanists. Then comes extraction via alcohol or methanol, then fractionation into different fractions.After this the fractions mainly chloroform and methanol are processed for column chromatography.On obtaining pure compound which is detected by TLC , spectroscopic analysis is being done which tells the details about the structure of the compound.

There is no such thing as "best technique" when it comes to plant metabolite analysis. You should optimize your own method specific to your plant samples. This includes sampling design, efficiency in the selection of extraction method, optimization of solvent system in chromatography, etc. For guidance, you should have a copy of the book "Phytochemical Methods - A Guide to Modern Techniques of Plant Analysis" by J.B. Harborne if your research is focused on natural products chemistry and phytochemistry. It includes techniques for phytochemical screening and compound profiling.