How do I know which one is the best forex broker?

First - always start with the platform of choice and features/add-ons/EAs that you’ll be using.
This should narrow down the number of brokerage agencies a bit.
Once that’s clear - time to look at the broker.
If you want to share your Forex Reviews then can take a look at [Only admins are allowed to see this link]

Regulated - Must be regulated with at least one big regulator. Always check with the regulator if that broker is indeed regulated or is just registered.
Spreads, fees and commissions (contract specifications) - depending on the type of trading strategy this (spreads) might not be a factor for some.
Account specifications - types of accounts, how to fund it, minimum amounts for funding and withdrawals and timeframes required for the transactions to go through. Also, the different ways you can fund and withdraw are important (bank transfer, card payments, payment providers like Skrill, for example)
Order execution - DMA/STP is essential but these days every broker will claim that and it’s a bit tricky to pin down until you’ve actually started a live account. One thing that you can do to determine if the broker is a genuine DMA/STP provider is to ask for a post-trade report (also called post-trade transparency report) - if they can provide you with such a report you can then see for yourself if the execution times are delayed at any point of the process.
Again, it’s almost impossible to verify this point without a funded live trading account!
No Dealing Desk - The market is hard enough to navigate without having to trade against your broker. This one is kind of explained in the point above.
Negative balance protection - self-explanatory
Online reviews and community feedback for the broker - want to take some time to research what your friends, colleagues and the internet in general and trading communities have to say about this particular broker.
The broker has been operating for at least 5 years.
Is Automated/Algorithmic and High-Frequency Trading (HFT) allowed - if not this could be a signal for internal issues and system/platform problems within the broker’s structure and potential trading and security vulnerabilities. If a broker’s systems can’t handle a high volume of trades that means his servers and systems are not good enough and thus may become an issue for all traders using his services.
10 Last but not least - Support response speed and knowledgeability. - You want a broker who has the support desk that will get involved with your issues and requests as fast as possible.