I'm having a hard time choosing between Michigan and Miami. I'm in the same boat as NDCrusader. My hate of Miami was forged in the height of the Catholics vs Convicts era; grade school & high school for me and when my Notre Dame fan identity was taking shape. The Miami rivalry has really fallen off of late, but that old bitterness remains. It still turns my stomach when I see someone wearing 'Cane swag.
As for Michigan, the gut-punch results of ND's recent games against them, the obnoxiousness of their fans, and my recent awareness of the ugly history the Irish have with them has really elevated them to top scumbag rivals.
Being an East Coast guy, Michigan never really registered for me in my early days of fandom. I personally encountered much more jag-off Miami fans in my life than Michigan pukes, versus what all you Midwest guys had to deal with growing up. It's still a tough call for me.