It's hard to say definitely because when you do trial there are periods of excellent execution and spreads, other periods maybe awful and unsatisfactory. I guess market conditions like liquidity and volatility have their say to. But still I would single out Hotforex or Tickmill as brokers of first choice to try.
I liked the trading conditions at FXOpen, which is regulated by the FCA. This company offers the possibility to open a full ECN account starting from only $100.
I have seen many past regulated brokers who turned into scam and not sure if the traders got their money back so better to check the brokers repute beside their regulations.