Whether you have 6 pac or 8 pac merely depends on your genes.
You cannot train abs and turn your 6 pac into an 8 pac. The shape
of abs is purely genetics and thus cannot be an indicator of
"ripness". Just look at Ronnie's abs, it's far from perfect. A good
indicator is to go for a body fat perecentage measurement using
callipers. Veins only show the vascularity of the individual.
Compare at Rich Gaspari and Shawn Ray. Rich is more vascular than
Ray but I believe both men have the same body fat percentage during
competition.
As for the best form of cardio- There isn't. There are a few
ways of doing it.
1) After gym session: Interval training- Warm up 5min + Sprint 1
min Jog 2 min x 5 + Cool down 5 min= 20min. This is recommended as
you are already warmed up and since your muscles have micro-tears
in them plus you are already glycogen depleted, the body will use
up your muscle for energy if you do traditional LONG SLOW
CARDIO.
2) Non training days: Long slow cardio in the morining- 30 mins.
Insulin levels are constant (low) in the morning before meal 1.
This will allow you to tap in fat reserves for energy and thus
allowing you to burn the fat.
P.S. Have you heard of circuit training? You should do them
cause it suits your goal. It's a high intensity cardio-lifting.