Agree with all you've said about worst and best, and try to avoid Toddler Group politics at all costs, (tho got last two mindees thru be recommended by mum's who met me there!)
Best is when
First time they don't cry at leaving mum,
rush in morning to tell you what they've been doing
accept some behaviour change youv'e been helping with (eg having hair brushed and styled, not 'showing off' when being picked up, learning to share)
Worst is when
they have been so used to having their own way
they can't accept any rules etc and getting them to participate drains all your energy and distracts from care of other mindees
you wake up feeling rotten but no outward symptoms, and one of the parents has the most important thing to do at work, or is dealing with family stuff and really needs support and you just have to get on with it til the last one goes and you can get on sofa with a blanket!! (not desperately ill obviously but needing lots TLC)