I went with 3rd-4th.
Going from the NES to Genesis and SNES was a huge leap not just in graphics, but presentation. There was more you could do with environments, effects, and more that have become standard. The music quality went up a lot and was really close to CD quality, especially the SNES. This is also the generation we went from 1 game portable systems to portable systems with interchangeable carts. It felt like console gaming on the go for the first time ever. This was also the generation that sparked "console wars" that nowadays is looked down on, but back then it was fun. This was also the generation we got ratings boards for games and got more standards for the industry.
Also this was the leap where it actually felt like you could play arcade games at home. While the SNES and Genesis did a decent job at this for most games it was the NEO GEO that really made that a reality. It showed that you can have the exact same experience at home while not having to go to an arcade. Let's not forget that this was the leap that technically made arcade sticks standard.
Speaking of arcade sticks, controllers were vastly improved in the leap also. The NES controller started the standard for what controllers should be like where the SNES perfected it. The SNES controller took what was good about the NES controller and improved on it. We went from 2 face buttons to the 4 button layout that is still used to this day. The left and right trigger buttons were also introduced making more buttons available without compromising the experience. The Genesis has 3 buttons and the super pad for that has 6, but I think we all can agree that it wasn't a good layout, and it showed when the triggers and 4 face buttons became the standard instead.
This was also the leap where we started adopting CDs into gaming. Of course it was mostly used for silly things, but this showed what CDs could do for gaming. It was the 4th-5th generation leap that really showed that CDs were better for gaming than carts. This argument can also be used as to why the 4th-5th leap was the biggest jump.
Of course every generation, or generation leap has downfalls, but I think the 3rd-4th gen leap in terms of what has become standard, improving on aspects of game design, music, creativity, and more is what led to more innovation for future generations. The 3rd generation was the spark while the 4th generation was the light that showed what was possible.