I think the absolute worst numbers are 17 and 23... but I also have somewhat of a distaste for 129. Let's not get started on the worst letters...
I got killed on the Iomega Zip Drive. I bought the standalone drive and a built-in drive for my home-built computer. I ended up spending hundreds of dollars on 100MB and 250MB disks that I never, ever used. Well I put backup data on them but I never did have the occasion to actually restore from them. WHat a waste of money. Now hard drives are so cheap that I just set up mirrored hard drives (RAID) to backup my data.