For ubuntu, you want Longsleep's Xenial Ubuntu Image. All Pine64 images are based on Longsleeps work at the kernel level, but that is where the story usually ends. However, he also put together the minimal Ubuntu image, and it will work with all three board variants (512MB/1GB/2GB).
Otherwise, have a look at the DietPi image for Pine64... here is the announcement for it on the forum. It is a heavily optimised debian jessie distro aimed at having the minimal possibly process and memory footprint, whilst having plenty of features and being easy to use.
Otherwise, have a look at the DietPi image for Pine64... here is the announcement for it on the forum. It is a heavily optimised debian jessie distro aimed at having the minimal possibly process and memory footprint, whilst having plenty of features and being easy to use.