See, I see 'abs' and 'core' as 2 different ball games. If someone's talking about 'abs', it's really quite simple. They either want to see them more, or they want them bigger. To see them more, get rid of some fat. As for getting them bigger? Squats and Deadlifts!
Training 'the core' is a completely different topic. The size or visibility of the 'abs' (a small part of the core) is an irrelevance. These discussions normally intertwine these 2 topics though, so it never really goes anywhere.
A better response, I think, when someone mentions this is, "so, are you talking abs, or are you talking about the whole core?". If they don't know there's a difference, then they'll learn something straight away.
However, here's an interesting approach. Let's just say Squats and DLs are the best exercises for increasing 'Ab' size. Therefore, they are superior to typical 'core' work like planks, roll outs, single leg stuff and Chop variations.
Well, all the 'core ' stuff will make your muscles work better together, and make both sides of your body work better together, which will enhacne your overall performance, and that should carryover to the squats and deadlifts... So you can improve your performance on 'the best ab exercises' by doing stuff that doesn't appear as good for abs. On top of that, you're far less likely to get injured if your train the 'core' properly.
There's really not a simple answer to anything. What's the best Pec exercise? What's the best trap exercise? Best back exercise? You'll never get people to agree. The answer is probably how you utilise a combination of options available to you. I mean, the laws of adaptation make these answers invalid anyway. How can Wide grip pull ups be best for my lats when I just adapt to the exercise anyway? They're not. But vertical pulling is.
To end my rant - I think people here the term "think movements, not muscles" and say, "ah, ok, that makes sense" then instead of writing down "chest/back", they'll write 'push/pull'. However, regardless of how they label their training split, they still think about things in terms of muscles.
I think most reading this thread think "my abs are a good size, I don't do any ab work, so squats and DL's work best". If you get away from that mindset, and think about movements, or function, you would think "well, i'm not doing anythign on one leg (single leg stuff), i'm not addressing each side of my core seperately (chops), I don't have anything that resists movement (plank variations and pallof press)".