I still think my absolute favorite is a good DKA. I can think of no other complaint where you can take someone on the brink of death, choose the right management (even if it is algorithmic) and have them discharged within 24 hours absolutely healthy.
On the flip side, some of my least favorite are asymptomatic hypertension and hyperglycemia. The only reason they dragged their non-compliant butts into the ED is because they checked their numbers for the first time in 3 months. I just can't get over the fact that even though it's easy to treat these folks, I know that in 24 hours (or less) they're going to be right back where they started.
"Oh... your sugar is 500... maybe you should take your damn medicine."