My own experience is that clamping is the most effective.
This is the routine I used to gain about 0.5” EG over 18 months:
-Warmup
-5min standard jelqing
-10min clamping
-5min slow, powerful jelqing
-5min Sadsak Slinkies
-10 more minutes of clamping
-Warmdown
This was a really effective routine for me, but I’m glad to be off it now. It’s pretty brutal to the penis, resulted in a lot of discoloration (going back to normal now, 7 months later), and required a lot of porn for me to stay properly engorged.