Based on what exactly?
I'm not saying Mayor Pete can win the Dem nomination; I'm saying he would be the nominee in the best position to defeat Trump.
The Dems, to win, need to perform a balancing act so as not to lose centrists while retaining/inspiring the youth vote and suburban women. Buttigieg is by far the most articulate of the bunch, isn't a fossil like most of the rest, and plays a moderate on television well enough to flip states like Pennsylvania.
Sanders will inspire the youth vote but will turn off the moderates. Bloomberg may appeal to centrists, but he's not going to bring out the youth vote, and will likely turn off women. Buttigieg's the most formidable rival for Trump because he stands the best chance of pulling together the traditional Dem constituencies while appealing to independents and those seeking a "sane" alternative to Trump. And Trump will find it harder to attack Buttigieg, while Sanders and Bloomie present obvious, easy targets.