Who wrote first, Mark, Matthew or Luke?
How about Marcion? (I have a video about Marcian priority in my queue, no joke, but haven't found time to watch it yet.)
Seriously,I heard the most convincing arguments for Mark. Just on the face of it, it is the simplest and least myth-ified one, which makes it a likely candidate. Matthew being the second candidate. There are some serious scholars in favour of Mt priority.
I'd have to look it all up to take part in a detailed debate though, this is just off the top of my head.
I would like to know the gospel of Paul. Damn *******, how dare he never write one?
As for Q, sure - but when we talk about "the oldest gospel", I presume we talk about the oldest one we actually possess.