Drop sets
Pyramids ..probably the same as ascending-descending sets & ladders. Different part of the country may use different names for the same thing.
Partials
Rest-pause
Cheat reps
one & one half system
Forced reps
Static holds
SS's, tri sets & quad sets.
Giant sets Usually 6 to 9 or 10 exercises in a cycle..non stop.
Breathing 20 rep squats, cleans or DL's
All of the above could be listed as intense, in one form or another. May find it can be just as much a mental effort as a physical one on most of those systems.
Try most of those on an experimental bases since I got serious when around 16.
Most productive, for my body type anyway;
Rest-Pause
Partials
Cheats
Breathing 20 rep squats.
The hardest, but worth the sweat...20 rep breathing squats. Adding more weight each workout.
Good Luck.