For the past few decades I have grown a variety of Haricot Vert called 'Contender'. It's a tried and tested variety that is easy to grow and produces well.
However, I have recently been informed about 'Crockett', and have already bought my pack of seeds for 2019.
I'm told there is no comparison to other varieties, both in flavour and overall quality.
A friend who grew them this year (a famed horticulturist), told me she would never grow any other variety ever again. That's as good a recommendation as I need.