"Rightful Heir" is a solidly written, well-acted piece about faith and religion. The themes are personally relevant and timeless for everyone, and the viewer immediately connects with the story and is drawn in. In fact, the script is good enough to hold its own were it to be used for any dramatic series, not just Star Trek.
What makes the story really shine are two things- first, spectacular sets, costumes, and camera work give this Klingon oriented episode the look & feel of a feature film; one has to really watch it twice to catch all the visual delights the producers painstakingly worked into it. The wide variety and intricate detail of the Klingon costumes is particularly outstanding.
Second, the excellent casting of veteran character actor Kevin Conway in the lead role; his in-depth, powerful, yet sensitive performance as Kahless is another reason why "Rightful Heir" has a high quality, feature film atmosphere. Conway's resume' is quite extensive but he is perhaps best known for his role as "Sgt. 'Buster' Kilrain" in "Gettysburg"(1993) and "Gods and Generals"(2003).
Maybe this isn't the most action packed or memorable installment of the series, but it certainly ranks up there as one of the finest produced in every respect.