Wikipedia itself says Larry Sanger "co-founded" Wikipedia, but I don't quite understand why. If you get into the details, he was Wales' employee at the time, and made initial version of Wikipedia while being paid as such. So I'm tentatively with Wales on that ATM.
"founder" is a weasel word that doesn't belong on Wikipedia.
Employing someone doesn't let you pass off their achievements as your own.
If Wales had anyone else, or had gone it alone, it's unlikely Wikipedia would be what it is today.