Samsung 850Pro for speed and reliability. 10 years warranty.
But since your question is more about SSD rather than iMac, I suggest you check any of the many SSD review sites.
Only technical requirement is that it is 2.5" SATA (provided you are talking about putting it internal and not on Thunderbolt or so). See my signature on how to update your iMac to USB 3 at the same time. You'll also see the 850 Pro.