This is tough to answer... I have 4 kids: almost 12 (son), 10 1/2 (daughter), 2 1/2 (daughter), and 1 (daughter)... So, the older two are close in age, and the younger two are close in age.
I'm having difficulty with my 10-year-old daughter, right now; she thinks she's the 'boss,' of everyone! Not just her sibs, but of mom and dad, too, and that just doesn't jive well with me. No way, so she and I are clashing like nobody's business. :) My oldest---my son---is a breeze, most of the time, although his perfectionism can drive me nuts, sometimes.
My 1-year-old is getting into everything; climbing up on stuff, scaring me, so I'm up on my feet, a lot, keeping her safe... My 2-year-old throws a lot of tantrums, and is going through the not listening stage. Oh, the fun of those 'terrible two's'... I take heart, though, cause the good news is, the 'horrible three's' are on the way! ;)
So, right now, the 10-year-old stage, and the 2-year-old stage, are kicking my ***... They can be really sweet kids, and they're all awesome, but when they get on one of their trips, it's stressful.
*edit* Sorry, I only answered half of the question... The easiest stage, was the baby stage. Before they were mobile; when having mama was enough to keep them satisfied!
Married, with 4 kids.