For me, I really take time to research about the team and the purpose of the coin, what is the coin about, is it gonna be a hit to people, cause once people find it useful and they see their advantages upon buying the coin, definitely its gonna be a great hit and its time for you to invest in it. Having a great team really tells if the coin is gonna be something, so take time studying and knowing about the team. I check for a good dev team that communicates with the community and comes through on their promises.
I usually see some new Altcoins that I want to invest, whether the Altcoin is worth investing or not. I usually go to the market and see some new Altcoins that have small price. Usually, a new Altcoin who price has less then 1 satoshi is good for investment. That is because those price of Altcoins usually will increase sooner or later and I will get profit. The profit will depend on the increasing price, it maybe become 100% profit, 300% profit, or even 1000% profit if I am lucky. But not all Altcoins will give us profit, some of them may go down and disappear. So we need to invest in some new Altcoins and not just one Altcoin. Anyway, that is just my opinion. The best Altcoin to invest is definitely Ethereum.