#1 Citadel
#2 Augustine
#3 Terra Volta
I don't care about merits per hour, for me the rating is by how much fun i can have, a tf either has diverse enemies, maps or locations and makes it a good run, some tfs like numina certainly aren't marathon worthy, but i enjoy my occasional numis.
Citadel hits 1# brown tunnels and council make for a very boring time, I only managed to do 1 with it being the weekly and i immediately logged off when finished and played something else.
Augustine is a chore, but it isn't tied to an accolade that i consider necessary the way citadel is. that's pretty much the tie-breaker between the two.
Terra Volta is on here because of the reactor, it isnt #2 or #1 because sometimes a good team makes this tf fun, but I don't love running in circles killing stuff for that long, not my idea of a good time.
Honorable mention Synapse - it is boring, but it is a beginner lvl tf, good for xp on alts, good for multiple badges, easy as it gets, i can solo it leasurely with any character, no power requirements there.