GSkill RAM has consistently been at the top of any memory comparisons going back years. The actual memory modules are made by companies like Samsung, SKHynix, Elpidia and Micron. GSkill then integrates those modules into the DIMM that plugs into your motherboard. Through high end manufacturing processes their respective memory usually outclocks the competition.
I'm not sure Corsair even makes their own DIMMs, I believe they outsource everything.
OCZ does not and has not ever made DRAM modules. ( Edit: and hasn't existed as a company for several years )
Kingston was the last company by 4 or 5 years to sell DDR3 1600 that needed 1.65v. They're good at marketing.
All the good high speed/low latency DDR4 uses Samsung B-die ICs. GSkill is just better integrating it into a DIMM than other companies.