IMHO,
Of those 4, I would say that the best is MLB . Good coverage of a sport that ESPN undercovers and a good mix of live games. Good off-season material. Then would come NHL. Again, in-depth coverage of a grossly undercovered sport and good live games, during the regular season, plus non-NHL hockey (amateurs, minors, international) and good off-season material. Week during the playoffs, however. NFL is OK, but the only live games are throwaways that would be on regionally on a Sunday afternoon otherwise, mostly to small areas of the country. Off-season material is worthless however, and you can get all the NFL news you really need on ESPN or other general outlets. Last is the NBA. I suppose it is intended for people for whom the 60% of ESPN news and talk time devoted to the NBA is insufficient. Worthless.
However, there are not 4 "niche" channel, there are 7. The best of the 7 clearly is the Golf Channel. In depth coverage of a sport that ESPN, et al, cannot devote much time to, with great coverage of early PGA rounds, 4 round coverage ofNationwide, Champions, Euro, and LPGA tours along with some 4 round coverage of minor PGA events. Excelent talk shows, informative news, and great filmed deals like Big Break. Tennis Channel is similar, although I'm not a big tennis fan. Good coverage of a sport that you are not going to find elsewhere, outside of its majors. The worst of those 3, and IMHO, it has declined greatly, is SPEED. Turning from a good general coverage of NASCAR and other forms of motorsports, to a collection of infomercials and canned programming having nothing to do with auto racing.