How can I find the most studied SNP of a gene?
Can you provide the gene of interest? 

You can usually search Entrez for the gene symbol in the dbSNP database, which will pull up a NCBI Entrez page for SNPs located in the gene and will also give an indication of their clinical significance (benign or otherwise). 

Alternatively, if you have a specific disease in mind, you can check more specific databases for a SNP, such as COSMIC for cancer-related SNPs.

Lastly, merely Googling the gene of interest (for example, "EZH2 SNP") in Google Scholar will bring up articles that present SNPs of interest. 

Some combination of those methods should yield what you want, but I might be able to provide a better answer with more info.