Just posted in and reading the 'worst thing that's happened to you' and am now in floods of tears.
So thought I'd start a converse thread. 
The best thing that's happened to me was ds being born. A botched abortion and years of infertility meant I thought I'd not conceive.
The nicest thing anyone had ever done I think is a neighbour who I've only ever meet twice paying for me, Dh and ds to fly back home when I'd learned my DF had leukaemia.