I used to sonsider the Ring to be the most objective, thus most credible one. However, after it was purchased by a promoter - GBP that notion went right down the toilet.
A belt only holds as much meaning as we give it. As someone said, when Lennox unified the titles then was stripped, whatever belt he had became 'the belt', whether it was the WBC, The Ring, or just from Calvin Klein - it was LENNOX that had to be beat because he was the real champion. Just like how Kelly Pavlik is the lineal champ even though he doesn't have all the belts that Bernard Hopkins had when he gave them up to Jermain Taylor.
No single title is really more credible, although the Ring belt seems better. You really have to base this on a boxer by boxer basis.