I was taught that [私]{わた(く)し}, [僕]{ぼく}, and 俺{おれ} are the most common first-person pronouns.
Recently, I stumbled upon the word [我]{われ}, which supposedly means the same thing. An online dictionary lists all of them as common. Since I haven't heard about 我 until now, I wonder how common it is. What is 我, and how does it differ from the other common first-person pronouns?
Which is the right plural form: [我々]{われわれ} or 我ら{われら}?
我 (also written 吾) is an older way to say "I", not as common as the other ways you listed but still in use, particularly by older men.
I asked my friend when one would hear it, and she gave the context of a older guy giving a speech in a company setting, where the assumed tone would be masculine and slightly formal (this is a subjective opinion). Perhaps indicative of it's provenance, the kanji of 我 is the way to say "I" in Chinese, rather than 私.
Both 我々 and 我ら are used, and these can also be used to refer to oneself as well as the plural form. Other ways to say "I" are うち、わし、せっしゃ、おら、おいどん and [我輩]{わがはい}.
My friend uses 我輩 as a joke sometimes, as it sounds very archaic. I remember asking a friend about 我 and was told specific situations where 我 would be a natural alternative to 私, but I do not remember the answer, and perhaps someone with more knowledge will explain this aspect of the word (for example someone of a specific occupation using this word to refer to themselves). What I do remember being told was that it would not be a good word for a young foreigner to use as a first-person pronoun, as the nuances behind it would just be awkward.
Its primary use in the Tokyo dialect is first person pronoun, but note that in the Kansai dialects, it is more naturally used as second person pronoun, a variant of 自分, usually when threatening.
舐めとんか、ワレ。
ワレ、なんぼのもんじゃい。
