I'm with my boy Grant, thank you very much.
Ahem, but I'll answer the questions anyway.
First, which creator do you think had more of a impact on comics and who do you think is the most creative.
Both Moore and Miller seriously outclass Gaiman in this area. I'd say it's Miller, for better or worse. Watchmen was a seminal book but outside of that, I don't think Moore's other books had the same impact on the industry. Nothing to do with their quality of course. But with Miller we have: Daredevil, Ronin, Wolverine, Dark Knight Returns, Year One, Born Again, Sin City, Hard Boiled, 300, etc. His influence wasn't necessarily good (see: the 90s) and he's a crazy old fuck but he has my eternal respect.
Most creative: Moore. He's the pre-Grant Morrison and he's better at non-superhero stuff.
Second, you have a chance to have one of those three men write a comic based on your idea, which man do you pick?
Gaiman. I don't want to talk to either Moore or Miller. At best, I'd send them a note thanking them for their work, which they would probably set on fire.
Side note: the only Sandman that matters is Sandman Mystery Theater.