Bittrex has probably the most number of coins to choose from so best exchange from that point.
For some smaller altcoins not on bittrex, use cryptopia.
GDAX(Coinbase) is good for transferring your fiat currency in and buy BTC/ETH/LTC with it. After that, send that
BTC/ETH/LTC over to bittrex and buy whatever altcoins you want. That's how I'm using it mostly.
If i understood what i read correctly there is now a way on bitterx to exchange one coin for another which i believe is really just selling one by them on the open market and purchasing another coin you requested. i will see if i can find where i thought i read that. I looked at quite a few sites this am so hopefully im not confusing that.
Here it is. If you click on "the lab" drop down it will talk about "any2any".
Any2Any allows you to easily convert one digital currency to another by market selling one currency and market buying the other.
The price is not guaranteed. It is an estimate based on the current orderbook
You may have some dust left over from the trades
The standard trade fees apply
Any2Any is being retooled. Thanks for your feedback!