I thought something was a bit off with the numbers given in the announcment
Striker Fragments per dungeon
Code:
               	WI	KotS	BF	UW	NC
Normal     	10	10	10	10	16
Hard        	12	12	12	12	18
Madness   	14	14	14	14	20
Sands of time cost
Code:
Normal     	4	4	4	4	6
Hard        	4	4	4	5	7
Madness   	4	4	5	5	8
Which gives us number of fragments per SoT refresh (120 SoT)
Code:
               	WI	KotS	BF	UW	NC
Normal     	300	300	300	300	320
Hard        	360	360	360	288	309
Madness   	420	420	336	336	300
So, discounting other benefits of each level, the best levels to grind striker fragments are obviously, from best to worst
Code:
420   4-SoT Madness
360   4-SoT Hard
336   5-Sot Madness
320   Night City Normal
309   Night City Hard
300   Night City Madness / 4-SoT Normal
288   Underworld Hard
I'd say just go for Powerslave.