It depends on one's specific interests and investment goals.
Apart from HDFC AMC, some other popular stocks within the HDFC group are HDFC BANK, HDFC and HDFC LIFE INSURANCE.
You can see the full list of HDFC stocks here.
You can also dig deeper into HDFC group stocks by taking a look at their valuations and leverage ratios, among other parameters on Equitymaster's Indian stock screener. We have a dedicated screen for top HDFC group companies...
You can also see the list of our most popular screens here...