Hi Pietro,
brokers trading against you is something of the past, and I've been in the FX market since 2001, when EUR/USD spread was 5 pips, LOL... If you stay away from CFDs which, as some have already pointed out, are basically bookmakers trading against you, any regulated broker, even on a MT4 platform, are ECN/DMA shops that will make money on a per-trade commission basis. You've got to choose wisely, as some charge unreal commissions per million traded which eat away your profits if you're a scalper. Other than that, I don't see any chance of a spot FX regulated broker taking the other side of your bet and running their book. Why should they anyway? More overhead, position risk, etc...all negatives in today's world.
Swiss/European/UK/Middle Eastern regulated brokers (spot FX) will work alright for you.
Hope this helps.
NVO
Not anymore. Retail MT4 platforms nowadays have aggregated liquidity pools that are better than "the big boys" platforms. First-hand experience, not just hearsay. I have traded tickets of 5M better in MT4 than some of the institutionals (Currenex, Hotspot, etc).