Which power Bank do you guys use??

Same as title, I am looking for a portable charger for my mobile phone, I saw from ebay there are millions, the price goes vary from $5-$55, it's a big price range, a friend told me that cheaper power bank usually are not really safe since their battery cell is not stable and probably low quality, so I would like to know which brand do you guys will recommend and where can I buy it ???

Best,