Im a believer in one or 2 days a week hitting abbs with heavier weight on a machine. If your lean enough, the abbs can grow bigger just like any other muscle. Some abbs are just bigger than others even if the 2 people are the same bodyfat. Like the 1st post on here the guy has big abbs. Bigger than many others thats why the pop out more.
I think really anyone can build great abs. i think it takes a combo of a few thing. 1 day per week heavy wieghted ab movemnets and 1 day per week bw stuff with higher reps. Once the abs are built up then i feel they can be maintained with copmound exercises squat deads etc.