Kanter hasn't played in a long time. Also, his game seems to be developing more as a forward than as a center. There are a few teams that have Jonas higher than Kanter on their draft boards.
Biyombo hasn't had the success that Jonas has. He came out of nowhere and doesn't have much of a resume. He might not even be the age he claims to be. His offense is not just raw, but completely undeveloped. I don't think anyone is expecting Biyombo to be picked before Jonas.
Of the three Jonas is more of a known commodity. He has also had the most success in actual competition.
In the end, I guess it all comes down to what you think the direction of the franchise should be. Personally, I think that the Raptors
should solve their issues at the center position before any improvement can be seen on defense, or in the standings. Picking any of Jonas, Kanter or Biyombo would be a good start.
Others prioritize other positions first. I think that's a mistake, since good PGs and SFs can be had by trade or free agency. Bargs/Amir/Davis could land us a good starting PG, but I doubt they could land us a good starting center. There just aren't that many, and teams that have an extra one usually prefer to hang on to them.