I need more information to understand. I don't like most of them, but there are some great ones too. I mostly know tennis these days. Bad ones: Chris Evert and John McEnroe. I liked both as players, but I don't care to listen to them. There are so many good ones, though. Brad Gilbert is one of the best on the men's side. The women in particular have a stellar bunch: Mary Joe Fernández, Rennae Stubbs, Lindsay Davenport, Mary Carillo, Martina Navratilova, Pam Shriver, Chanda Rubin. The funny thing is that most of them are considered the "B" crew and do the less marquee matches or are on the Tennis Channel. The key to a lot of it is knowing when not to talk. Major networks, like ESPN, seem to not like silence and talk and talk and talk, while the Tennis Channel lets an entire point happen in silence, for instance, which I find refreshing. Lindsay Davenport is especially adept at that. I've watched matches with her commenting when she lets 2 and 3 points played without saying a word. Then she'll hit you with some key knowledge and insight.
Baseball has a lot of good ones. I don't watch hardly any football, but Tony Romo is surprisingly good and can explain the intricacies and insider playbook type information. I don't know how people watch football. They're always talking, filling air time with actual filler. Maybe the general public doesn't like to be left to their own thoughts?