Intelligence is the most important stat, at least end-game. I guarantee you're not going to hit that Eternal Overlord medal without it unless you enjoy grinding for a year.
Here's why the other stats aren't as important as Intelligence. Once you're at The Creator and can reliably defeat him by just spamming Dark attacks, that is you have enough Willpower such that you don't run out and you have enough Vitality he doesn't kill you, then the only thing you should ever continue to improve is Intelligence.
Optionally you can boost Speed some if you just want to dominate, but once you're the one attacking The Creator first, you no longer should even optionally boost that.
And Strength is a joke by that point. Are you going to be dealing 4000+ damage with each hit with your melee weapon? I don't think so! But you can easily hit that spamming Dark attacks if you equip all the wizard stuff and just concentrate stat points on Intelligence.
Really, you can use and improve Strength early on if you want, and using the better swords and such, but by late game you really should be full wizard gear and concentrating everything on Intelligence.
Also, for leveling, you'll want to fight easier enemies -- a lot -- before strong enough to take the next one, etc... The game is a lot of grind.