Are the Ferengi the richest and most powerful species in the quadrant?
This is a 2 part question.
Are the Ferengi the richest species?
Individually most likely yes. we have very little actual levels of wealth, but someone such as the Grand Nagus would be massively wealthy. far more then anyone in the Federation, and most likely far more wealthy then any single individual in both the Klingon and Romulan empires as well.
As the Ferengi Alliance Most likely no, especially since the alliance as a governing body is essentially administrators keeping all individual Ferengi inline with the Rules of Acquisition. The Federation, Klingon and Romulan empires most likely would have controlled far vaster supply of resources at a governing level then the Ferengi Alliance did.
Are the Ferengi the most powerful species in the quadrant?
Definitely not. The Ferengi Alliance is more of an overseeing body that directs Ferengi commerce, not a military organization.
We never have counts of total wealth from any major power, as well as accurate military strength, though we know the Klingons, Romulans, and Federation are all similar enough to keep any faction from starting a war with another. But because of the vastness of the other 3 major powers, and the focus on the individual with the Ferengi, it shouldn't be hard to see the 3 major powers as also the 3 "richest" powers as well.
The Ferengi are also a very peace orientated power in the quadrant, they are not looking to gain military power, only personal wealth. The Ferengi are used during negotiations as they have never been part of a Interstellar war (according to Quark in DS9: The Jem'Hadar) They were able to maintain this neutrality during the quadrant wide Dominion War as well.The Ferengi's drive for money has allowed them to stay relevant in tech with the other races though, and showing that the ships the do posses are capable of defending themselves, or defeating Klingon, Romulan, and federation ships.
The neutral tendencies of the Ferengi and their government were evident in the 34th and 35th Rules of Acquisition: "War is good for business" and "Peace is good for business."