I think the best way to find good altcoin is by looking at the trading volume, trading exchanges, and the market capitalization history. You have to study the altcoin growth history carefully including userbase, community, and how good the team of developers are. I think finding good altcoin is easy but people are not doing the proper market research.
I agree with you indeed we have to do it all so we can know which is good altcoin and which are not,
but doing research like that does require exact time and also has to be patient
The top coins would cause the newer people a steadier feeling in investing because of all of those factors listed above. I think it's best for investors to look at the top coins only.