I think beldre would be the one 'cause:
1. Tindwly was dead during the reformation of the world and has very less chances to be counted as an orignator.
2. Arianelle has been metioned by name by marsai rather than title for one, and wax refers to breeze as his ancestor but not "Lady Truth".
3. "Lord Mistborn" and "Lady Truth" has a nice ring to it.