What is a colloquial/slang expression for a cigarette in Brazilian Portuguese?
I only know that the formal one is "cigarro m".
"Cigarro" is both formal and colloquial.
Some slang words and phrases are:
"pito", "crivo", "de branco" ([dressed] in white) - any cigarette
"mata-rato" (rat poison), "estoura-peito" (breast burster) - cheap, strong, bad cigarette
"guimba", "bituca", "bagana", "ponta" - a cigarette butt (or a joint).
Edited - also, "as vinte" (the twenty) - a cigarette butt.
(Edited because a cigarette butt is not a joint.)
"cigarro" is the most common way to refer to cigarettes in pt=BR, in both formal and informal environments. There are several other names but none of them are widely used throughout the country. Some are slang words restricted to a specific group, some are regional usage. The dictionaries mention quite a few of them but, to be honest, I never heard them.
p.s. I've heard, however, the words "guimba" (a joint) and "mata-rato" (very low-quality cigarette), as mentioned by Luis Henrique in his answer.
In addition to the other answers, one may also use the word "pigas (m)" (despite the "s", this is the singular form), as in
Me vê um pigas? (May I have a cigarette?)
I believe this is specific to the portuguese as spoken in the city of São Paulo.
Also, my friends and I sometimes use the word "cigarette", but with a Portuguese pronunciation, something like "cigaréti", but I've never heard this outside this specific group of friends.
Both words, "pigas" and "cigarette", refer to tobacco cigarettes.