Depending on your interest, and don't know what you've seen before...
The West Wing is fantastic and there are 7 seasons of joyful telly to get through. Deadwood I've heard is amazing and is complete, but with only 3 seasons. Sopranos is 6 seasons long, I have that lined up, saved for a rainy day, that was supposed to be really rather good. I confess to being a great big fan of NCIS a formulaic crime show, now in its 10th season but the dynamic and writing between the main cast really make it fun to watch. At 4+ seasons Burn Notice is also fun - though I've only seen one or two episodes so far and it has cult B movie hero Bruce Campbell in it! Lie To Me was ok for its run too. There's a load more
Currently I'm watching Dexter and love it - it's on its 7th season and though I've just seen season 6 is wonderfully entertaining. House? If you haven't seen that, there are 8 seasons ( though the last two definitely a bit weaker ). Buffy? or the star treks must have a few seasons for a sci-fi similar to battlestar, or possibly doctor who if you're into it?