We have studied the host galaxies of a sample of radio-loud active galactic nuclei (AGNs) spanning more than 4 decades in the energy output of the nucleus. The core sample includes 40 low-power sources (BL Lac objects) and 22 high-power sources (radio-loud quasars) spanning the redshift range 0.15<~z<~0.5, all imaged with the high spatial resolution of the Hubble Space Telescope. All of the sources are found to lie in luminous elliptical galaxies, which follow the Kormendy relation for normal ellipticals. A very shallow trend is detected between nuclear brightness (corrected for beaming) and host galaxy luminosity. Black hole masses are estimated for the entire sample, using both the bulge luminosity-black hole mass and the velocity dispersion-black hole mass relations for local galaxies. The latter involves a new method using the host galaxy morphological parameters μe and re to infer the velocity dispersion σ via the fundamental plane correlation. Both methods indicate that the entire sample of radio-loud AGNs are powered by very massive central black holes, with MBH~108-1010 Msolar. Eddington ratios range from L/LEdd~2×10-4 to ~1, with the high-power sources having higher Eddington ratios than the low-power sources. Overall, radio-loud AGNs appear to span a very large range in accretion efficiency, which is all but independent of the mass of the host galaxy.