EHpro and Hcigar make the best Kayfun/Russian clones.
Tobeco comes in a close 3rd
Fasttech is really hit and miss, plus there seems to be some strange sort of Kayfun clone from fasttech that has 4 juice channels instead of 2 that's really prone to leaking and flooding.
And just so you know....
The Kayfun 3.1 is the same as the Russian. They have adjustable airflow, a collector tank under the coil, and adjustable 510 connector.
The Kayfun Lite lacks the adjustable airflow, collector tank, and adjustable 510 connector.
The Kayfun Lite Plus is the same as the Russian 91%. It has adjustable airflow and an adjustable 510 connector, but lacks the collector tank. Also, while the airflow is adjustable, the screw is on the bottom which requires removing the topper from your mod to adjust it.