Formal:
* Bench: This is a common term, especially in legal contexts, referring to the group of judges presiding over a court.
* Panel: Often used for a group of judges hearing an appeal.
Informal:
* Jury: While technically referring to a group of citizens deciding a case, it can be used informally for a group of judges.
* Conclave: This refers to a gathering of judges, particularly for a specific purpose, like a judicial conference.
Ultimately, the best collective noun for judges depends on the specific context and the desired level of formality.