ECN brokers I think are the best as they are the liuidity providers and they directly approve the trades. In other kind of brokers the trade is passed by broker to the liuidity provider and then they approve the trade at a given price and tell to broker. It is a long process.

There is not much difference in ECN and STP. Usually STP brokers are also ECN. But if we rank then ECN will be on top.

I dont think anyone will like Dealing-Desk brokers. Both STP and ECN are NDD type of brokers. So in terms of transparency and execution, there is no difference between STP and ECN broker.

STP brokers transfer orders from traders directly to banks (liquidity providers).
ECN brokers provide a competitive platform for all types of traders i.e banks, individual traders and market makers. So an ECN broker is able to, and must, show the true depth of market.

Dealing Desk brokers are the best. No requotes, no slippage etc.

No dealing desk brokers earn only if a trader looses. So to remain in business, they can take some "hidden" steps which will decrease profitability or increase losses of a trader. 

If you are a small trader then something like a spreadbet firm is the best - small spreads, instant execution, and most importantly tax free (if UK resident) which offsets any stop hunts that may take place.  Otherwise something like an ECN broker if you are a large trader and need to move large volumes.  Anything in between is inferior imo, but can be used.

Fixed spread 

No dealing desk 

ECN 

These are the thing that every trader look is a broker and nearl all of brokers are offring these, Now many traders are moving toward ECN because there they can see the market order and can decide more accurately and this help them in their analysis prepration also.