@AddiG I agree! I didn't like the first season that much either, it felt like they were just trying to create a horror series.. But since they actually came up with a plot and made the characters much more interesting it's been the best series ever in my opinion!
I really can't name my favourite seasons, there's been so many good ones.. I think season 10 had a nice plot, but I really loved the fourth season too. It's easier to name the not-so-good -seasons haha :D I love Cas so I think the seasons in which he's got a lot of screen time are great :)