Fortunately in recent years regulators did a great job in forcing unauthorized providers to went out of business. There are still a lot of scam brokers, but definitely much less than few years ago and that's a great thing! First place when you choose a broker to trade with, you should find and confirm if they are regulated and never ever trade with unregulated, suspicious companies, no matter what kind of offers they give you!
Dukascopy is the best example of highly reputable and regulated by multiple authorities broker with top level customer services and highest review ratings! The best thing about Dukascopy is that they have powerful platform for trading Forex, CFD and Crypto-assets on one place. You can find here more
CFD brokers ratings.