As the title suggests, does a best quality shield grant a +15 or a +25 to parry?

I don't see why it wouldn't be +25; Parry is a Weapon Skill test, so the Best Craftsmanship bonus would apply, as well as the Defensive bonus.

I can't think of anything that should ever give you a 5 in the bonus anyways.

19 minutes ago, ThenDoctor said:

I can't think of anything that should ever give you a 5 in the bonus anyways.

Defensive quality give +15. Corebook, p.145

5 hours ago, Jargal said:

Defensive quality give +15. Corebook, p.145

+15 and -10.

I can remember only one quality giving +10 on a common test without drawbacks: Balanced. Adjustments that apply to the specific tests by the opponent may be greater (Fast gives -20), but these are situational, not the character's choice.