I’ve been searching for figurative synonyms in French for “leader/s,” “one of/among the largest” and/or [member/s of the] “top tier/echelon” (some of the, imo, good and more easily understood and translatable English alternatives to “bulge bracket,” as suggested in a comment by another member) to try to find at least one that could be used idiomatically, perhaps with any domain but, for the purposes of trying to answer your good question, with that of “investment banking” in particular.”
That search has led me to an article in the finance section of LesEchos.fr (“Chase Manhattan négocie la reprise de la banque britannique Fleming” by PIERRE DE GASQUET) which contains, in the second sentence of paragraph 5, the author’s following parenthetical gloss/translation (with emphasis added) of “bulge bracket”:
Mais, même après cette acquisition, les analystes de la City restent
sceptiques sur ses capacités à rivaliser avec le fameux « bulge
bracket » (catégorie des poids lourds) composé de Merrill Lynch,
Goldman Sachs, JP Morgan ou Morgan Stanley Dean Witter.
I find the two notions (bulge bracket in English and French’s “catégorie des poids lourds” [“heavy weight (class)” in English]) similar enough in both their register and in their respective figurative natures for you to consider “Les poids lourds” to capture the domain-neutral, “top-tier” part of “bulge bracket” ....
... and when combined with “du secteur bancaire”, the domain could be narrowed down to “the banking industry [in general].”
To further narrow the domain to “investment banking industry,” you could consider:
“du secteur des banques de financement et d'investissement” or BFI for short (from Cafedelabourse.com) to address @Laure ‘s good point that the notion of “investment financing” is essential here,
or just “du secteur des banques d’investissement” if you feel that including “de financement” still renders the term too general for your purposes, as the linked Wikipédia article mentions as a possibility.
(Please note that Mr. Gasquet begins the cited article by speaking of “des … fleurons de la banque d'affaires …,” where “fleuron” could be seen as meaning “top-tier” and therefore perhaps “heavy weight,” but I think “fleuron” is much too pretty of a word to use for these “bulge bracket” banks and that “poids lourds” is much more fitting.)