Damn, that's too many for me. At least in the IT industry.
BlackBerry, if they knew how to capitalize their first successful years in the smartphone era and managed to adapt in those post-iPhone years.
Nokia, if they knew how to capitalize their dominant position in the pre-smartphone era and understood their old phones were already in their last days.
Commodore International, if Jack Tramiel put more focus on the personal computer side of the business instead of being busy selling calculators.
Be Inc, if Apple made the deal with Jean-Louis Gassée instead of Steve Jobs, thus BeOS becoming the base for the new Mac OS instead of NeXTSTEP.
Seattle Computer Products, if IBM made the deal with Gary Kildall instead of Bill Gates, thus CP/M becoming the main OS in the IBM 5150 instead of DOS (PC-DOS).