For me, it's not as simple as that. You have to consider how much money that you have, how long are you gonna hold that coins/tokens and so on. Diversification is important, but if you choose just because of the market cap, you'll probably miss big gains if you buy coins that have a low price.

For example, with $100, you should look for coins that have a low price if you want to get 5x-10x (but choose wisely). If you buy Bitcoin or Ethereum with it, you can't expect you'll get $200 at the end of the month.