Visited the link you posted. There are so many different GARCHs, which one should I use if I want to test SPY?Why not learn them? They were made by very smart people who I am sure you can learn a lot from. Each of them try and capture a slightly different feature of how vol moves. You are searching for the "Aha!" moment right? That moment might come from reading one line in the "OLIGARCH" paper.
https://vlab.stern.nyu.edu/en/ this website calculates them for you. GARCH is better than historical vol and worse than implied vol for liquid assets.
