the PSOne series H is the best in my mind cause I've made joysticks that adapt well for Xbox and Gcube.
DC & USB work on any Sony Pad. PS2 is a given.
If you want to just use a digital pad, I've found that the sony digital series H actually have exposed copper points/contacts on the PCB to solder into. (even thought they are small)
Don't forget that DC adaptors don't see the Select, R2 and L2....
I have a toggle switch that swaps positions of my L1 R2 buttons so that i have a capcom 6 for DC gaming on one of my creations. (capcom frame perfect stick)
It's the best one to hack because the main chips are on there with the analog sticks. There was another board (dualshock) that I was playing around with where I desoldered the analog stick board because they were on a daughter board and the encoder was screwed after that. It kept detecting that one of the sticks was on, one way over... USELESS now...
Moral of this story, you can ditch the digital daughter board cause of the ribbon cable that goes to it can be hacked into, but you can't ditch the analog sticks... if your controller has the sticks on a daughter board.
This all makes sense if you understand that the dualshocks had two styles. One that had the encoder chips on the digital side and one that had the encoder on the analog sticks, side. The later is the prefered. (PSOne Series H... the white one)