Most helpful - staff who actually rolled up their sleeves, got down on the floor and interacted with my child and showed me how to interact. Nursery staff, TAs and Parents learn very little from advisory staff who come in, sit in a corner and criticise what you are doing but are not prepared to demonstrate how to do it themselves.
Least helpful - pretty much everything else.
I suppose top of my list would be wasting time on appointments with countless professionals in every sector who were allocated to help, support or advise me - only to find out that their actual remit was to find ways not to help, support or advise me so as to not spend any money. I would rather they had turned up and said 'we will only be able to visit your son once this term so lets get through as much as we can even though its vastly inadequate'.
when what they actually did was waste hours of my time defending why he didn't need help because he was too bright / not far enough behind on enough measures / didn't exactly meet the criteria / we ware asking too soon and hadn't exhausted all the options before them (even when it was obvious those options were totally inadequate) / they wouldn't do it because 'thats what DLA is for' / passing the buck to another dept.
If they don't want / can't provide something, just say so; but don't lie and spend hours of everyones time making up reasons why he doesn't need it.
if the time spent in meetings, appointments, responding to complaints and tribunal planning had been spent delivering actual intervention my son would have had about 200 hours of provision in the year before tribunal instead of the 1 hour he needed up with, and they would have had a better chance of winning.
And my other pet hate - speech therapists who say we don't do direct speech therapy because we think its better if the primary care giver works with the child - and then when your child gets to school suddenly the primary care giver is excluded from everything as 'they only work with the school not the parent'. Either I have a crucial role in my child learning to speak or I don't.