You should ask yourself which altcoin are you going to trade, because not all exchanges have the same list of these coins and maybe an exchange can be useless for you. I would prefer to start with poloniex, take a look also on yobit and others too.
I prefer Yobit.net. yobit almost listed all major and new coins in the listed and also very easy to trade and buy sell.
it have the feature of trade major coins with fiat also.
like bitcoin, litecoins. etherium etc.
for altcoin trading yobit is probably the best choice.
polenix and bittrex also good but I didn't used them much.
Yobit is much faster than poloniex when it comes to list new coins and I found yobit faster also when withdrawing the coins, poloniex needs more time (requires more confirmation) when depositing altcoins but its faster when depositing bitcoin (it needs only 1 confirmation comparing to yobit which needs 2 confirmations)