+1 on hockey. There's just not time to describe what's going on adequately. Football has enough stops-and-starts to make it interesting, even basketball does to some extent (NCAA hoops can be entertaining on the radio).
I love baseball on the radio, it's perfect for the medium. Radio is referred to as the "theater of the mind" and the very best color commentators can make it so you feel like you're there so well that you don't need a picture... they paint one for you. I'd argue that, at times, baseball on the radio beats watching it on TV. Maybe because 100% of the time you know that your announcers will be local and will know your team. Watching games on ESPN or other national networks can be excruciating as they try to pretend they know about a player just because they've read his stat sheet.
Second and third place for excruciating radio sports? NASCAR (or any auto racing, for that matter) and, yes, golf.
In honor of the upcoming Olympics, I don't imagine gymnastics, diving or synchronized swimming would be very interesting on the radio, either. Or how about the marathon, smog or no smog?
