Apple computers tend to be a bit picky about things like memory timing being rigidly in spec and so forth, so avoid any bargain brands.
Kingston, Crucial and OWC are all fine (OWC often buys its actual RAM chips for its modules form Samsung) - but go onto their web site and get the module part numbers for the modules specifically guaranteed to work in your machine. Each company has a web feature to do that - look up your specific model of Apple computer and they will tell you exactly which module they make that is certified to work in that machine. Do not buy Kingston's value RAM modules - stick with the Kingston premium stuff guaranteed for your machine.
P.S. Kingston and Crucial actually make their own RAM (not just the modules, but the actual RAM chips themselves). Samsung also makes its own name branded RAM. OWC does not manufacture its own RAM or modules, but they do spec them out and then test them inhouse so they can guarantee they work as described on their web site.
You are better off with iMacs using high quality RAM. Kingston "Value Ram", by the way, has repeatedly been reported here to be unstable in iMacs. iMacs can function unstablely with non-matched and cheap RAM. Buyer beware. Many Users in this Forum have repeatedly reported the success and excellent prices and customer service of the following online vendor (I always use them too):
Hope this helps