Marketcap is a popular way of finding coins to invest most people just pick the ones with the largest marketcap and invest on said coins however aside from the marketcap it is also widely advisable for investors to look into different factors
First check the white paper of the project in here this gives you insight about how the project would work it gives you its goals and other important details you need to know about the project
Second of course you need to check the developers or anyone behind the project it is most likely that a project backed up by credible and trustworthy people would have good potential
There are still more ways to determine good altcoins like checking their liquidity or trading volume and whatnot what I mentioned are just a few of them
I would say marketcap is definitely a good way to start, and the way the project has bene handled since release is also quite important as well as checking the community. I have seen plenty of projects with a community that only talked great things about it, only to search it on twitter to see plenty of complaints that got banned at the telegram group for stating their complaints. Remember, you can go into a group of ten thousand people, all singing praising songs of a token, and that may not be the reality, that might be just ten thousand that didn't get banned, always check reddit and mainly twitter, you will find the truth there.
Between marketcap, the project creators and the community, you will get your answers for any project that you may want. I have invested into so many different projects so far in my life, and when all three are great, it's rare that a project would do bad, not impossible it could still do bad, but it is certainly not that common at all.