Sites like bitmex with a 100x leverage offers trading from as little as 50,000 satoshi, this may be a good way for you to get into trading initially. Sites like yobit.net and poloniex offer altcoin trading, as for the best coin I wouldn't know, I tend to stay away from altcoin trading.
Instead of yobit and bitfinex you try direct cash for trading like localbitcoins, Where you trade instantly very fast and smooth with the verified users. Get one account with good trust and trade volume and go for it.
This idea seems to be safer than trading with altcoins. I also try doing this kind of trade because the risk is not that high all you have to think or watch is the current price of USD and BTC