4-5 for sure, although I'd say the interesting question is which was the biggest leap aside from that one?

For me, after 4-5 it's between:

2-3, mostly because of Nintendo basically saving the industry at that point and pushing game design forward in such huge ways, or

6-7 - not the biggest step up from a technical standpoint, but certainly a period that saw some massive change in game design (motion control, touch-based input, the rise of mobile gaming, the rise of indie games, online gaming approaching mainstream popularity).

It's hard to tell, but I'm between the leap from gen 4 to gen 5 or the leap from gen 5 to gen 6.

Well aware of this. I'm the kinda guy who took a History of Gaming class in College.

I would go with 5 - 6 just because how big Ps2 was and everything the it brought with it... + it wasn't a bad DVD payer either.

Gen 5, hardware accelerated 3D is the biggest jump, everything before that was just improvements upon 2D, everything since has just been improvements upon 3D.

The 5th gen was hands down the biggest single jump! There's really no contest

Definitely 5th gen by a country mile.

I'm going to assume 9th gen will hopefully be a lot different than 8th in which we see a much bigger leap along with VR being the norm across 2-3 platforms instead of just one, that and some AR and all 3 being neck and neck instead of 1. (bad competition when there is just 1).

This one was tough, but I said 5 to 6. Sure there was some 3D earlier, but non that was all that smooth or great. At 6th one could really see the potential to realistic 3D games like we have today (and going forward.) But 6-7 was another big one. I think 7-8 will just keep getting better.

1 through 4 seemed kind of meh. Yea, things got better, but it wasn't the revolution the hype was.

While it had been tried before, the 7th gave us movement that worked, which was a big deal.

So would Pong and Super Pong be gen 1?

I do not know, I gamed on PC till 2012.

I think the 9th gen will be like the 6th gen. Right now in gen 8, PS4 is basically taking the place of the PS1 and the N64, ushering in the next era of gaming. We will have some games that stand the test of time, and some games that are amazing at launch, but only set the foundation for much better games in the future. 

I think PS5 and VR will be the next PS2. 80% market share, many exclusives, fully realized VR gaming, but still not as awesome as PS4 and VR due to just being refinments to a way of play already evolutionized on PS4.

As for the (). Having one leader puts the focus on games, and thats what its all about. Gen 7 was a nightmare with all three systems within ~10M units. I nope that never happens again.