I'm about to tally the stuff up around the community. Here goes:
12 Epoch
3 Masamune
10 Time Egg
4 Ocean Palace
20 Mammon Machine
7 FATE
19 Pendant
0 Sapling
75 votes total.
And a goodie piece of discussion elsewhere:
If you're talking about the Guru's inventions, the Mammon Machine has the most impact on the story I believe. The Ocean Palace ws built so that the Mammon Machine could draw more energy from Lavos. Therefore, no Mammon Machine, no Ocean Palace. I'd also like to point out that the Ruby Knife was also created for the sole purpose of destroying the Mammon Machine. Alot of good it did though. But while Lavos would still eventually rise up and destroy the planet, Zeal would have prospered as long as the Sun Stone held out. If it wasn't for the Mammon Machine waking Lavos, Zeal would've never had fallen. And with that being said, history, as well as the whole story, would be greatly changed. Not to mention that the Mammon Machine became Zeal's crest. And if Zeal didn't fall in the way it did, there'd still be wizards in Crono's era. I do believe that Lavos was responsible for robbing the survivors of their magic. Hell, he d**n near drained Magus's and he still didn't get all his spells back (Geyser and the Elemental Sheilding).
If you want to know which Guru had the most impact per say though, I'd have to say Gasper. Residing in the End of Time, I do believe that he can see the timeline as it stands and changes. I got this impression from what Spiekko said when you first meet him about seeing a multitude of wars and such. And existing outside of time, he is unaffected and therefore, aware of any and all changes in the timeline. I believe that he partially manipulated Crono and crew into changing certain events in time and in effect, acted as a entity. Take the quest to kill Magus at his castle for example. I believe he knew that Magus died in the attempt to summon and kill Lavos himself. And that's why he didn't tell them anything about Magus not actually creating Lavos. He knew that Crono and crew's interfereance would change history and Magus would be spared. And while Gasper doesn't have complete control over what happens to the timeline after the changes are made, he still has an idea of what will happen and can decide on what course of action Crono's party should make. In short, Lucca's understanding of space and time wouldn't have been enough. Without Gasper's guidance, Crono and crew wouldn't have gotten as far as they did.