While the Grand Nagus role did not seem to inherently confer great wealth (that is, there doesn't seem to be a huge cash prize that persists after the position is gone... we've seen temporary Nagus position holders who went back to, more or less, their original station after the fact) the powers that came with the position would almost certainly enable any long-term Nagus to become rich beyond anybody else.
When Quark thought he was going to become the Nagus in the episode "The Dogs of War", we see these exchanges:
QUARK: I don't. And I'm going to do something about it. This disease has got to be stopped before it destroys everything Ferenginar stands for. If Zek wants me to be Nagus, he's going to have to let me do things my way. Or else, I'll turn down the job!
ROM: Are you serious?
QUARK: You bet I am.
ROM: But being Nagus will make you rich.
QUARK: I don't care. I won't preside over the demise of Ferengi civilisation. Not me. The line has to be drawn here. This far and no further!
And
QUARK: Just think. This time next week I'll be living in the Nagal Residence.
NOG: They say even the waste extraction fixtures are plated with latinum.
QUARK: That's right. Oh, the first thing I'm going to do is rip them out and replace them with solid latinum fixtures.
LEETA: That's a little indulgent, don't you think?
QUARK: Of course, that's the point. A Nagus has to lead by example. My greed has to be a shining light to everyone. A testament to the rewards of avarice.
So clearly it's a position that inevitably leads to vastly increased wealth.
It's easy to see why. The Grand Nagus is allowed to have his own business interests and, with no apparent conflict of interest laws in evidence, can both designate trade territories and overrule the Ferengi Commerce Authority (creators of virtually the only laws the Ferengis have to abide by) on many of its decisions. In addition, bribery seems to be an accepted part of Ferengi society... in exchange for granting a business opportunity, for example, Quark, in his brief tenure as Grand Nagus in "The Nagus", was offered and accepted a 50/50 split of the profits. That was the profits of the entire synthehol trade in the Gamma Quadrant. Even asking the Nagus advice (and there's a steady stream of petitioners) requires a payment.
With all of this unchecked power, it's hard to imagine the Grand Nagus, given enough time to use it, not eventually becoming the wealthiest Ferengi, unless he wasn't trying very hard, which was probably one of the few things that might get him deposed.
So, Zek, the Grand Nagus for most of the run of Deep Space 9, was almost certainly the richest Ferengi we ever saw in material terms. It's quite possible he was the richest Ferengi overall in that time period, but it's difficult to confirm as there doesn't seem to be an explicit reference.
But, in the spirit of the season this question and answer was posted, Rom, who succeeded him, was probably the richest Ferengi in Star Trek, for he had learned there were more important things in life than merely the pursuit of profit.