High school is the best. Preferably 10/11th.

Elementary: smallest school, you may have the same kids all day, you probably don't specialize (you teach a bunch of subjects). In my case, the ES kids I taught were at a Gifted/Talented international school, so... there was no behavioral issues that needed anything but simple structure. I taught Kinder, so even though they were great kids, they needed a lot of structure. I also taught 3rd grade. They were super easy. I don't like ES for a few reasons: 1) I don't like having the same kids all day or even half the day; I like class changes! 2) They're super needy and I'm not very nurturing, 3) They don't get sarcasm yet, and that's half of what I say.

Middle school: I never taught 6th (see my thoughts on ES), I don't like 7th, and I enjoyed 8th well enough. In middle and high school, you generally have class periods (90 minute blocks or 40-60 minute periods depending on school) and see a bunch of classes a day. You generally specialize in a subject or a couple subjects. The kids need a lot of structure in middle school. Behavior can be rough. Disrespect and hyperactivity can be problems consistently. Everything needs to be broken into small parts, even for the smart kids. Focus is not great as hormones are raging. They can be quirky and fun, though. A lot of work compared to high school.

High school: 9th graders are pretty much like 8th graders, but 10th/11th/the first semester of 12th are easier. You get more apathy but a lot less outward disrespect/defiance/hyperactivity/disruption. A lot of kids will just skip if they're going to disrupt. They'll fail, but they're less likely to bring others down with them. And most kids get slightly more serious about school than middle school (because "it counts"). Slightly. Don't see that as a miraculous super-caring. Plus, they all get sarcasm and they have actual fully formed personalities. So, I like it best. I've never taught a full class of seniors, and I know those teachers get off a month early, but I think they earn it with the rest of 2nd semester, because 2nd semester 12th graders are a special kind of awful sometimes. But mostly, high school is pretty easy compared to middle school.