Know most: high energy astrophysics/observation, stellar nucleosynthesis
Most interested: high energy astrophysics particularly black holes and AGN/cosmic rays/neutrinos.
Know least: constellations (because we use a boring, if better, naming system for stars :P) and radio astronomy (because it involved fourier transforms which I hate and long boring lectures about antennae, which I hate more).
Know the most about: Stellar dynamics/celestial mechanics. That's how stars or other objects move under the influence of their mutual gravity and/or the gravity of other objects. Basically the dynamics of everything from asteroids and their mini-satellites to clusters of galaxies. Fun if you like that sort of thing.
Most interested: Dynamics of dense stellar systems. Globular clusters and galactic nuclei and how they work in other words.
Know least: Cosmology I suppose. All the make-it-up-as-you-go-along pseudo-science just puts me off.
Know the most: Distant galaxies, especially interested in large globular masses and their interactions as they collide (even though we can see but a split instant of the time it takes these collisions to occur). Know the least: Every bit of it (particularly since there is so much speculation involved in learning about places we can't go and observe first hand [my kingdom for a rocket ship:)]). Still I like to study and conjecture as much as the next guy, it's the next best thing to knowing the truth.
What's your area of astronomy on which you know the most about, mines the solar system.
What's your area of astronomy on which you find the most fascinating, mines black holes.
What's your area of astronomy on which you need know the least about, mines stars and star constallations.