There are actually lots of them offering margin trading if not to mention Poloniex. Among them are also CEX, Kraken, Bitfinex
Thats why I am a bit confused.
Start with Poloniex for first time. Cool graphs, low rate for loans.
I agree with you that if you start with Poloniex for the first time then it should be avoided because it is riskier if you do not need sufficient knowledge in margin trading especially for newcomers. The best exchange trading site for margin trading is Binance.
Choosing a platform for margin trading is the last step but you have to follow a lot of steps before it. You have to start gaining some knowledge about how margin trading works. I have seen a number of people going for margin trading with no previous knowledge just because they can have major profits but you should understand that without knowledge there are none profits.
You have to devote your time in learning more and more about the trade calls. You can never make a mistake by buying at a wrong price in margin trading as the price reversal might make all of your funds get liquidated in just a couple of seconds.