You are totally right that market cap is a horrible metric to "rank" cryptocurrencies, as it can be easily manipulated.
But the flawed rankings are caused partly because the ranking services (Coinmarketcap et al.) compare apples to bananas. Above all, because they compare complete blockchain projects with tokens which are created out of thin air, at almost no cost, on an existing blockchain.
For example, I would very much like a ranking service which compares only coins that have:
- an own blockchain (similar to the "Coin" table on CMC)
Coin market cap has this option. You just have to select from the dropdown menu whether you want only coins or all crypto according to their market capitalization. I agree with you though since the first ranking to appear on the site is list of top cryptos and not coins with their own blockchain.