Instead of searching for the best broker for scalping, I would suggest using a broker with good spreads like FP Markets or CMC that have stable spreads even when the marking is highly volatile.
How did you conclude that their spreads are good? Gathered bid-ask prices for a decently long span of time or judge by transaction costs of your trades? How many trades have you made with them?
If you study the information provided by the broker, you can choose the one that is most suitable for your trading.
Have you tried FP Markets? Their trading conditions are some of the best, with faster execution speed and spreads averaging at 0.0pips for most parts of the trade.
ECN brokers are best when it comes to scalping. For my recommendation, go for fxview as their commission is cheap ($1 per side) and low spreads on many forex pairs.
Establish connections with all brokers. You can then choose the best spreads, sometimes you can even get negative spreads
I personally prefer FP markets because of their good trading conditions and is suitability for scalping. The spreads begin at 0.0pips, with stable and consistent spreads even when the market is highly volatile during market events.
For me the most suitable broker for scalping is FP Markets because they have a robust trading condition with spreads starting at 0.0pips for most parts of the trade. When compared to other ECN broker FP Markets commission costs and slippages are lower.