I've played Northern League in Lancashire and Yorkshire League (in um... Yorkshire)
Of those 2, I'd say Yorkshire League was better. Have played against several Liverpool Comp teams - the Current Liverpool Comp league leaders have 2-3 of the Northern League team I did play fors ex-players - I wouldn't say they were stand out good to be honest but seem to be doing well in that league.
I have a close friend who plays for the reigning Bradford League champions. He's ex-minor counties and previously played Yorkshire League too. He says it's a strange set up - players are amongst the best he's played with or against, but the facilities are a bit crap - lot's of dodgy wickets, making for a different style of cricket to anything he'd played previously.
I'd always heard that the Birmingham League topped anything I've played in.