Trading Bitcoin and altcoins can be both profitable and fun. However, you always need to make sure that you do it on exchanges that are trustworthy, fast and secure. In addition, I personally like a nice user interface, with nice graphics and clean fonts. High trade volumes are also a must, as none of the above matter if the 24h trade volume is 100 BTC. As a disclaimer, at the moment there is no cryptocurrency exchange that fully guarantees the safety of the assets that you store in their wallets, so it is recommended to choose the ones that offer you the best security measures, such as two-factor authentication (2fa), IP limitations and login confirmation e-mails. It is also very important to set up a strong password that contains characters (both in lower and uppercase), numbers and symbols. What is more important is to never use the same password for all the exchanges, but I assume you already know this.
This being said, here are the three cryptocurrency exchanges that I use right now:
1. Bittrex is now the cryptocurrency exchange that I use for most of my trades.
PROS: 2fa, login confirmation e-mails, IP check (every time you try to login from a different IP address you must confirm your identity by clicking a link sent via e-mail), high trade volume and a very nice, clean user interface. They also offer some nice trading instrumens, such as the conditional trading (also known as stop-loss).
CONS: it usually takes a while until they list some new currencies, so this could be improved.
2. HitBTC is the cryptocurrency exchange that I use for trading the new-comers of the market.
PROS: early-adopters (short time needed to list the new currencies), a lot of USDT markets (you can exchange almost every altcoin directly into USDT), large trading volumes, 2fa, the possibility to buy bitcoin and altcoins by transferring USD to your HitBTC USD wallet.
CONS: the user interface is not my cup of cake. Also, in order to withdraw fiat (cash) you need to send them a scanned copy of a bank statement. Which in my case is kind of impossible, since my USD account is in Revolut, and Revolut doesn't send any hard-copy bank statement. In fact, they do not send a hard copy of anything.
3. Kraken is actually the cryptocurrency exchange that I use to buy Bitcoin and that I will (eventually) use to cash out some of my profits. They have a pretty straightforward fiat deposit and withdrawal system that allows you to do the above in a multitude of methods. As I consider their user interface is not that trading-friendly, I usually just buy Bitcoin with Kraken and after that I transfer it to my Bittrex wallet.
So these are the three exchanges I mostly use now. You may wonder why a big player like Poloniex is not present in my list? That is because lately I have heard a lot of complaints about long or even unprocessed withdrawals from Poloniex, so I now paused all the Poloniex trading and also withdrew all my currencies from the Poloniex wallets. I didn't experience any inconveniences with them, however I consider it's better to be very cautious.
I hope this was useful for you and also would love to hear from you what exchanges you use for the trades you make. Feel free to comment!