Level 0 is not Really One thing: JaronK's list of levels enjoys a great consensus, but only includes levels 1 to 6. That list mentions the Truenamer as basically broken and does not really fit the levels, which is sometimes called level 7 (or simply "truenamer level"), but beyond that there are only the six.
In addition, for the most part, the levels want to treat classes by their full range of potential, from naive construction without operations to a fully optimized construction. The idea is that for a given amount of optimizationTop-level classes are more versatile and powerful, and lower-level classes are less versatile and / or powerful. The main point here is that, in general, class variants and options are not considered separately in the list of levels, they only represent more or less optimization. There are some exceptions, for example, dungeoncrasher, but for classes that are already at level 1, an option that makes them more powerful is not considered really relevant.
That has not stopped people from talking about the idea of things that are even stronger than what we consider to be level 1. The psionicist is remarkable for having dramatically more versatility than a regular craftsman, which is already an incredibly versatile class, even by the standards of the level 1 classes. That's remarkable. Sometimes, the psionic artificer is called level 0, or as suggested by the link offer: "If there is a zero level, Psy Arty deserves it".
The scholar of spell to power is also a frequent candidate for level 0 status. Again, we are talking about a character who has much more access to spells than a cleric or assistant, plus most of the power access of a psion. This thread has several people who call it level 0, for example.
But ultimately, none of these things have even remotely The consensus that has the list of main levels. Level 0 is not really a coded thing, it's just an occasional conversation topic. The psionic artificer and the scholar of the power of the spell are some of the most common options for those conversations. I do not think you'll find many people who say "do not, that It is not level 0! " even though.