In my opinion the best Forex broker online is the one that offers the lowest spreads. This is essentially the bottom line of Forex trading. You can chase customer service all day long or worry about the effectiveness of your trading platform, but the bottom line is the best thing and most important thing that you need to trade Forex effectively is profits, and profits are best increased by a low spread. So to decide the best Forex broker online I primarily look at the spreads.
This does not mean that customer service trading platforms and other factors do not come into the equation. It simply means that the number one thing I am looking at is the spread. I think that anyone else who looks at anything other than this is just mistaken. People who trade Forex are primarily in it to make money, so give yourself the best chance of making a profit with the lowest spread.
Probably the lowest spread I've ever seen for every single currency pair I know of come from FXCM. They are constantly offering one pip spreads that allow you to profit very easily and very quickly and Forex markets. This means you do not have to worry about getting dragged down by your broker. Your broker will not hold your profits back.
Most good brokers will offer this service only on very liquid currency pairs. This is because these are the only currency pairs they can afford offer this on. If several thousand people are constantly trading a particular pair they will still make a solid amount of money with one pip spread. Keep this in mind if you like to trade smaller currency pairs that you'll always be working against a higher spread.
I recommend finding a Forex broker that allows you to use metatrader. This is because this particular trading platform allows for very dynamic expert advisor services. These will give you automated Forex trading alerts that will tell you when and how to trade based on various market conditions.