"best" is subjective and contextual.
Attacking from top or the back is going to be much higher percentage simply because 1) you have gravity generally on your side, and 2) you have more positional control over what the other guy is doing. These are superior positions because of that. Guard is at best 50/50 neutral (or, depending on who is doing what, it might be something like 52/48 on either side or similar) and as such means there is a greater chance of the other guy being able to stop what you are doing.
But all of that is a general approach, and does not take into account the nuances of grappling. Someone can build a great guard game and be hugely successful with it - Roger Gracie, my professor, and Rudson Matteus come to immediate mind.
And the mention of Marcelo Garcia is a great illustration of this contextual idea. He doe snot do a lot of work from guard, but there is a very specific reason for that. Take a look at his build sometime. For his size/weight class he has incredibly stubby legs. He will always have a problem using guard against someone his weight division or bigger, so he avoids it as much as possible. The attack that "made him" as someone to pay attention to in the BJJ world in the early 2000's was an armdrag to a backtake. He would hit that on everyone and was rarely forced to use guard. Contrast that with a Roger Gracie who with his longer than normal legs makes his guard work very practical.
So don't think of it as either/or, but as something that may work for you or in a particular situation.