This is very difficult question. And very important. In short, invest in the first 10 currencies in the top. Personally, I do. You can not put all the money on one currency, because it is too risky. The more your investment portfolio, the better for you!
Just remember that investing in the top coins of the market is not a guarantee at all that you are going to make any kind of profits, and if you do not believe me the only thing that you need to do is to look at what coins were in the top 10 in the years before and I'm sure that you'll find some coins that you do not recognize and that were revolutionary at the time but that have been left behind by new coins with better technology.
So if you decide to invest in the top 10 coins you need to always track any kind of development in any of those coins so you can readjust your portfolio in case one of those coins begins to go up very rapidly.