Well, BTC and ETH have the most liquidity. If you want to learn how to trade from a technical standpoint the best would be to try with forex and a demo account (I don't like demo accounts btw).

But obviously, you can trade any coin with enough liquidity :)

Ah, and I forget to mention that I recommend trading 'only' 10% of your portfolio to avoid (in some manner) FOMO and FUD sentiments.

Good luck :)