All Forex traders have a favorite Forex tool or indicator that they find essential for their trading. While the answer might be different 
from trader to trader, the basic premise is that you will find that one thing that helps your trading through and through as it has consistently proven itself. If you don't have one, don't worry – you'll find one sooner or later.
For myself, I have always found the most important Forex tool is simple support and resistance. While many other traders will rave about a specific indicator, I find that these simple lines are like signposts on the highway – they show where we have been, and where we are going.
Quite often when I look at a potential trade, I will check the higher time frames to make sure the coast is clear. I cannot tell you how many times doing this has saved me. If you are not aware of the major support and resistance areas, it is quite easy to buy a currency pair right below massive resistance, for example.
Perhaps that massive resistance hasn't been tested lately and you are trading off of the 15 minute chart. Will you be aware of it? Of course not, because you are not looking at the "?big picture", which is absolutely essential if you are going to be successful. It is because of this problem I began paying close attention to major support and resistance areas as drawn on the weekly charts, and to relying on this critical Forex tool.
When I first started, it felt like a conspiracy. I would place a trade to buy, and the market would inevitably fall as soon as I placed the order. It took me a long time to understand that there are certain levels that price will react to over and over. If you don't believe me, look at the 1.0000 level in USD/CAD, the 1.30 level in EUR/USD, or the 1.0000 and 0.8000 levels in AUD/USD. I just wrote that from memory, that is how important support and resistance is for me.
I will often hear people complaining about how support might get broken. That's fine, because I also understand that when one of these major areas give way, it means something important has changed and we are heading in the other direction. Taking a small loss for that kind of information is a trade I will take all day as the loss might be 1%, but the future gains will be much, much more over time.
Because of this potential for profit, I cannot live without support and resistance. In fact, most trading systems use some kind of variation of the two anyway – so why don't you test them out for yourself?