Marvel makes some amazing comics and they have had no shortage of super powerful characters. Finding one that is stronger than all others is nearly impossible because someone will always have some sort of argument as to why their favorite is the strongest.
As far as coolest characters go, I always like to see characters who have ascended to a new level of power. Cosmic Ghost Rider is the strongest version of Ghost Rider. Cosmic Spider Man is the strongest version of Spider Man. But my all time favorite ascended character is God King Doom. I think that Victor Von Doom is probably Marvel's best villain/anti-hero however you choose to look at him.