Okay, here goes...
The best sex toy I think I ever made was actually for my girlfriend at the time and not for me. She enjoyed occasional vibes, suction, and estim and I had read about this DIY toy that would do all at once, so I made one and we gave it a try a few times.
It was a conductive clitoral suction toy attached to the top of a Hitachi wand with a small rubber ball inside that would jump around on her clit when the wand was turned on.
The wand was adjustable through a power tool speed control, the cup was attached to a pulsating electric vacuum pump, and the cup was connected to an ET312 with a plug in her vagina.
It was positively mad-scientist diabolical and made her go completely nuts and was definitely fun to use on her, but setting it all up was a chore and it lost its charm after a few times. If we'd had a permanent dungeon setup or something it'd been easier to have around. I don't have it anymore but still have the important bits to make another if I wanted to.