The easiest would probably be adults. They know what they want to learn. For adults they need to learn how to prepare for the test such as Ielts. As far as teaching to a kindergarten class you can introduce new games to them. A popular game that I played was hot potato but make sure you turn your back away from them when you stop the music so you show them that your not looking. Teach them phonics and get them excited about reading. 

My best experience was with about grade 3-5, which is guess would be 8-10 years old in the western reckoning. They've still got the childlike wonder, but they're a bit more clever and understand how a classroom works, but they still haven't developed a trace of teenage snarkiness.

Unfortunately, when teaching children of any age, it's impossible to keep resource teachers out of the classrooms, translating everything and not giving the students the oportunity to think. And completely undermining you as a teacher.

Teaching Chinese adults English is like trying to teach Hitler the Talmud. But at least there's no fucking in-class translator.

For me, in the states, teaching adults is much easier. I'm not teaching them a language, however. In another month, I will be volunteering with an ESL program here at home to teach immigrants of all ages English, so that might completely change what I am about to say.

Having taken foreign languages myself at different ages, it was easier for me to learn as a child than it was as an adult. And most of my foreign language teachers have said that teaching a foreign language to children is a lot easier than it is trying to teach an adult. 
Adults have their native language and many of its nuances and rules down pat. They have to deconstruct all the rules that have been pounded into their head, and relearn simple sentence structures, meanings of words, etc.

This being said, ESL isn't new to many Chinese adults. At this stage, many have been learning it since a young age. As many have said on this site, a lot are pretty confident in what they think they know (even if it's incorrect ie Kchur and the slang thread). For more experienced teachers, I think teaching adults a second language could be more challenging and rewarding.

For someone with little to no teaching experience, it might be easier to teach children. Teaching them words, phrases, simple sentences, numbers, etc might be a little less overwhelming to the new teacher. HOWEVER, you have to be very active and very animated to keep their little attention spans. 

I was thinking of teaching Kinder, but I've been thinking I might go a little older, like Kchur said, because they'll have a better understanding how a classroom works.

No matter what age group you decide to teach, it's going to present its own challenges.

My favorite age is about 6-8 years old, for similar reasons as kchur listed.  Plus at that age they will probably adore you if you are reasonably friendly. By the time they're 10 or 11 a lot of them are brats. 

I also enjoyed teaching middle schoolers, not because middle schoolers are awesome, but because their painful awkwardness was amusing to me!

For six months I taught a class of 3 year olds.  (This was all in Taiwan, by the way, but I assume it's pretty similar.) Although you do have to do a lot of songs and games, I found that expectations for Taiwanese kids that age were much higher than they are in the U.S.  The 5 year olds at my school could write English sentences better than native speaking American students were expected to write when I was a kid. 

And I totally agree about the annoyance of Chinese teachers translating your every word. 

University English majors.

And just about any university student when there is a CET coming up.

The students that pay lessons out of his or her pocket and have been saving to do so.

Kindergartens pay more because it's usually a nightmare job. I like teaching college aged people and middle aged businessmen. All my favorite students have been from those 2 age groups. 

I have taught college age, 3-5 graders, and 3-6 year olds. I would say that teaching the college students is the best, because even though a lot of them don't seem to be paying attention, the ones that do are really great. They are curious, and they know enough English that they can get their point across and learn new words. As for the most fun, that would have to be the 3-6 year olds, but then again, I only "taught" them for an hour at a time, and all we did was play games and learn a few words. As for the 3-5 graders, they're a bunch of brats. Some of them are very cute and sweet, but there are a bunch of them that are just a ton of trouble, and make it too stressful to be enjoyable. It's also more awkward, because they are learning simple English but still can't really use it to communicate with you. Sometimes I think that book is so useless in the beginning, they don't seem to get much out of it.

For me, I will not teach below College.  I really do not like kids much. I can tolerate them for short periods of time, but if I had to teach them daily, I would commit "sepukku" with a spoon and hang myself from a bonsai....

I prefer teaching in the workplace, as the students can see the benefits to the learning, through increased salaries and promotion. They are also actually getting paid to learn, instead of paying ridiculous sums to a school that doesn't care.

The audience depends upon the teacher.  Some find it easy to teach little children.  Others prefer adults.  Most would say, I think, that high school students can be tethersome to put it politely.  I myself prefer the little ones but they are not an easy audience for all teachers.

I won't go below seniors in high school. I like to have intelligent conversations about the world. From that age, they've formed opinions, have experienced some things (either directly or through various forms of media), etc..

One of my best students was a 16 year old, who is now in the US on exchange... and she's doing wonderfully!!

But, I think it really does come down to why the person is doing it. Sure, kids will just see it as another class, but as they get older, it starts to become more personal. So, as mentioned, English majors at uni, or business people who are there of their own accord.

Depends on the teacher just as much as the student. I've had a taste of all age groups, and they all have their issues. The only thing that matters is whether or not they believe they have better things to do than be in your class, and to a degree a little further than some would be comfortable to admit, that depends on how engaging your class.

I remember back to my high school and middle school days, my interest and consequent performance depended on the teacher rather than the subject. I would go from loving to hating and loving Chemistry, Physics, Geography, French and History just depending on the teacher. My favorite subjects Psychology and Biology were those, in hindsight, I had the luck of consistently having teachers who made it interesting to me.

I've taught all levels and once, I would have said adults because they're so much more motivated. At present I'm actually enjoying my kindy classes more than anything, which is odd because I used to hate kindy.