Welcome.

I have started on Bitstamp because it is considered as a reliable exchange. But it cant offer variety of trading pairs.
For the past year had no any issues with it, but to be fair did not deposited/withdraw fiat from it, but lot of my pals did and so far everything is fine.

For different altcoins you can check Binance, Huobi, Kucoin.
I can leave my comments only about Binance. For a time of being there had no issues at all - pretty fast and user friendly interface, place/cancel orders without delays, convenient mobile app.