An interesting topic indeed to ponder. Since the postulate is single level only, I'm going to have to go with, oh, 80.
Funny, though, because I more often use 20 than 80, I also use 0.2 a LOT, and not infrequently I use 300. The reason I'm going with 80 for this specific question is because I wouldn't want to be stuck with a single level that is just plain inadequate for SOME significant tasks, even if it is plenty for MOST tasks. So my take-home is that I am really glad to have a three level light (OK, a whole bunch of three level lights, but let's not harp on my insecurities
).