I like Danny Higginbotham, he clearly knows his stuff beyond just the big PL clubs, and demonstrates knowledge of the football league and European football regularly.
It seems like a lot of pundits just wing it without any sort of research. The soccer saturday lot in particular. Their knowledge of current football is shocking and has been for years. Higginbotham is the antithesis of that.
Hargreaves has a lot of knowledge of the game too, but he's not particularly good at articulating himself. Higginbotham, despite his knowledge, suffers from just being a tad on the dull side too.
I think Neville and Carragher are probably the best, in the way they balance knowledge and understanding with being entertaining and engaging.