For me, you've got to be great at a sustained level for a long time, not just dominant when you are fit and able to play. The sport's history is rife with on-their-best-day players -- ask any oldster about Lew Hoad in his healthy prime -- so I give the nod to Steffi Graf, who won each major at least four times, completed a Golden Grand Slam in 1988 and finished with 22 major singles titles. Tennis is the one sport that, because of so many historical factors that discount the tyranny of mere numbers, should and does allow for such a subjective view of "The Greatest"; after all, some believe that Pancho Gonzalez, with just two major titles to his name, was the best ever. But I'll go with Graf.
Best? Here's where I think there's at least a conversation to be had. Qualitatively, I maintain that no female player has performed at a higher level. Optional homework: Spark up YouTube. Then watch some of Serena's matches. Then watch some of the other candidates play. It's like comparing Albert Pujols to Rogers Hornsby, Tiger Woods to Sam Snead. It's barely the same sport. This will sound sacrilegious and is not meant to diminish the other candidates, but -- even accounting for everything from technology to training and more Gatorade flavors -- I firmly believe that Serena would beat any comers head-to-head. Surely that counts for something.
The biggest factor in her longevity is, of course, motivation. While Serena has always had "outside interests," she has also reached the inevitable conclusion that, while she might enjoy acting/designing/Portuguese, her true talent is hitting a yellow ball over the net with more force and accuracy than any other woman on the planet. And that, finally, is the engine the drives everything else.
On the other hand, I see Serena as precisely the type of player whose legacy will grow in retirement. Hers is a remarkable narrative, and with some detachment and time, it's easy to see this becoming a real sports legend. In X years, what will endure? A fit of pique against an official? A dubious withdrawal from a tournament? An ungracious remark? Or a family from Compton, Calif., that came to rule tennis? Also, in tennis, there is vast potential for image rehabilitation that can extend well into retirement (see: McEnroe, John). Given how smart and personable Serena can be when willing, she could very well win back fans for many years to come.
On the other hand, Serena should know better. While these types of outbursts may be forgivable at the beginning of her career, chalked up to immaturity, fans are less understanding now. That said, if she keeps playing, she'll have some time to fix these image problems. As we've seen in other sports, winning makes people forgive and forget pretty darn quick.