Of course bitcoin is the best one in terms of trading. Ethereum is a good one because of ICOs. I would propose you to go for ICO's. There are some incredible ICO's which will be the following best altcoins for their stage. Cartaxi is one of them, ARNA was a decent one, at the present time Electroneum is going on effectively. They will be the future in the altcoins and ICO cost will be too low when they succeed.
CarTaxi is definitely not one of them. Did you read their whitepaper? Does their model of using tokens really make sense to you? Or did you just read they compare themselves to Uber, so you think they'll be huge because of that?
There won't be any demand for their tokens because they will also accept other cryptocurrencies and fiat. So noone will bother using their phone to find some CarTaxi tokens on and exchange, buy them and transfer them to their CarTaxi wallet, so they could pay for towing when they could just use fiat currency.