Wow...Grampa versus Grandson.
In this case, I have to throw my opinion behind Gramps (Genghis.)
Both of the men had certain successes to lay claim to, but I think that Genghis' are greater in certain ways:
1) Genghis was the one who set the Mongol expansion in motion, while Kublai came into a situation in which the Mongols were already dominant. Genghis was the one who was able to unite the Mongols (by friendship or force.)
2) Genghis came up with the idea of utilizing his defeated enemies and incorporating them into his own forces rather than killing or disbanding them, making him much stronger.
3) Genghis began to use a system of promotion by merit rather than family ties, thus strengthening his army.
4) Genghis created the YASSA, the military/civilian law that governed people's lives,
5) Genghis delegated well in his military, trusting his loyal commanders rather than micromanaging them,
6) Genghis allowed a good deal of religious freedom in his empire, which reduced tensions.
In contrast, Kublai did some great things for the Mongols, but they were already dominant at this time and so came to play for a "winning team". His big claim to fame was making some big conquests for the empire and solidifying Mongol dominance of China. That was actually pretty important, but he came to power through what amounted to a quasi-civil war as he was not really the favored one to take over things.
So, in short, Genghis did more of the leg work than Kublai, and though both had successes, the reality is that Genghis' successes would exist even with no Kublai, but Kublai's successes could not have been possible without Genghis.
Thank-you. That really helped!