GDAX is very high-volume, US-based and pretty reliable.

If you're going to daytrade on GDAX - try Medved Trader, PC software trading platform. It connects to GDAX (and a bunch of other more conventional brokers) to pull down the data and uses it for the trader's tools - charts, tech. indicators, scans, alerts, etc. This kind of software is common for traders in regular markets, but is severely lacking for crypto trading. That's why we connected it to GDAX, so that crypto traders can utilize the same methods to trade that Forex traders can (since crypto is basically Forex).

Integrations with other exchanges (Bitfinex and HitBTC to start with) are coming shortly.

It doesn't cost anything to try - go to www.medvedtrader.com, register, download the software and run it.