Now, as someone who was born long after he retired, I didn't get to see him race, but obviously as an avid F1 fan I have heard the legends. It is generally accepted he is the greatest driver never to win an F1 WDC title however given that 27 more drivers have won the WDC since he retired, even having just one WDC is no longer as special as it used to be (ie, they had been 11 WDC years when he retired, compared to 69 since he retired.
However, it would be unkind to simply use that analysis - he was a very dominant driver.
For example, he is the driver with the most wins who has not been WDC despite racing far few races than any of them:
|10||Juan Pablo Montoya||2001–2006||95||7|
He was runner up in the WDC on four occasions, most famously against Mike Hawthorn when he personally protested against Hawthorns disqualification in the Portuguese Grand Prix despite that act ultimately handing Hawthorn the title (although this was not in the deciding race)
The other three runners up positions were to Fangio.
His percentage wins of 23.9% would put 10th in the highest percentage of winners (when you exclude Wallard and Vukovich who only competed in Indy 500 events so their stats are non comparable)
He was also incredibly highly rated by his peers to the point that even today, suggesting that he is not an all time great seems almost blasphemous.
I don't think there is any doubt that Stirling Moss was an exceptional driver, I think he was almost certainly a titan of his day - but in his day Formula 1 was a new sport, with no established history - he retired at the end of the 12th season. If we were to take just the last 12 years of Formula 1 and build an all time top 10 list, Button and Rosberg would be top 6 drivers - there's a good chance that Massa would be in it. Massa would be the driver that only just lost the WDC to the driver who went on to win 6 titles and the drivers with the highest number of wins who didn't become WDC.
Another detail to consider is that Moss raced during a time that motorsport was mixed discipline. You didn't just race F1, you raced everything. And in everything-but-F1 Moss's Wikipedia page is a constantly stream of yellow "1st"s - which again, affects how the legend is remembered.
When motorsport was multidiscipline the driver's reputation was based on their performance overall. These drivers would be racing Moss in Formula 1 at one weekend, and Lemans at the next. He'd be beating them most of the time and while Formula 1 wasn't important it wasn't the be-all-and-end-all. Today, driver's are single discipline and consequently - that's where it matters. Of course, this does bring up the thorny issue of comparing eras, and ultimately when motorsport went from multi discipline to single discipline is probably one of the more significant lines.
However, it could be argued that just as in other sports - such as tennis, where until Andy Murray won a grand slam there were always questions about whether he should be considered one of the sport's top four players, or just the best of the rest - that delivering on something is just as important as everyone knowing you were good enough to have done it if things had been different.
It's a curious question, and one that is difficult to answer as I suspect nearly everyone on this forum was born too late to have living memory of Moss's racing career. It's one I don't know the answer to, and by that, I mean I don't know what I think. I know that he was an exceptional driver. I know that his reputation continues to be strong in the sport - but I'm curious as to what others think about it...