How to Choose the Best Online Broker?

Check reviews such as the ones here and other reputable forums once you know what you are looking for in a broker. Then try then out on demo or with a small live account (that you can afford to lose).
Things to look out for are the commission charges, the spread and reviews on reputable forums. Also, if you're new to trading I would look into brokers that offer any educational support & resources.
Actually, you should comply with several rules in the process of choosing a broker I guess. It's kinda difficult and the main difficulty is to dedicate uncertain amount of time to the procces of choosing a suitable broker. Some traders don't want to do it and pick the very first broker they've found out. To my mind, it's very important to choose a proper broker, because brokr is a direc treflection of traders' success. If you have unproper broker, then you probably have much more chances to lose all your accumulations, that's how the psychology works. If you see that broker isn't interested in your success, then it's very sad for you.
There are several things that I would check for. At first glance, I wouldn't consider a broker that doesn't have a transparent website that lists their costs, spreads, the minimum to open an account, deposit and withdrawal methods, and so on. If those things are missing, it's usually a sign that the broker has high costs and unfavorable terms. Easy to reach support is another. Personally like to see LiveChat available as it seems like brokers that only respond via email take way too long to reply.
This site is great for broker reviews. Make sure that you do your research before you sign up and put your own money into it.
Define the best... There's simply no one. As others stated, there are a lot of ways to check if a broker is reliable, and I think that's all you really can do: do a background check and hope that your judgment was correct
I think you need to extensive research when selecting a new broker. Check out reviews on here and other forums. Try to speak to someone who actually uses them. I tend to lean toward the offshore brokers. They offer better leverage and the fees are low. I have never had any issues being with an unregulated broker but you need to research them individually.