Sunstone

Bad luck, all I ever seem to get are sunstones...

i think that it is a mix of player, location and time to decide the evolution item. sunstone is for me the most common beside upgrade

Probably bad luck. My frequency ranking is: metal coat, sun stone, dragon scale, king's rock, upgrade. The first four drop at roughly the same rate, while upgrades are a bit less common.

You are just unlucky, the sunstone is most common, the dragon scale is the rarest, at least according to data compiled over at Silph road.

Item distribution
Sun stone: 24.97%
Metal Coat: 23.12%
King's Rock: 22.31%
Dragon Scale: 14.74%
Upgrade: 14.87%

metal coat is the hardest in my area fallowed by dragon scale most common for me is upgrades

For me, nearly the inverse of the Silph Road distribution stats, so likely all RNJesus frowning on you:

Dragon scale - 9
King's Rock - 8
Metal coat - 8
Upgrade - 6
Sun stone - 5