My personal favorite season is 4. Eric Kripke planned it to be for 5 seasons, and it actually has an interesting season/series ender if they didn't renew it then.

Definitely watch 4 and if you still like it, watch 5. Then stop. For God's sake, stop.

And try to watch 'The French Mistake' that was hilarious and their 100th episode, 'Fan Fiction.'