Maybe you could try and ask this question on the Supernatural forum? I'd be interested to see what responses you get there.
Buffy gets my vote too, but as people have said, they're very different shows. Buffy is more of an ensemble, while Supernatural only has Sam and Dean. Basically the entire Supernatural mythology deals with Angels & Demons/ Heaven & Hell, and Buffy barely ever mentions religion, and obviously has a strong focus on vampires. Buffy is only based in Sunnydale, while Supernaturalis based wherever the hell the road leads them.
I'd have to agree with others that my main gripe with Supernatural is the melodramatic relationship between Sam and Dean. Buffy has more characters, and more humour, that allows this problem to occur less often, but with just Sam and Dean in Supernatural often when they fight I don't really feel invested because it's so over the top and I often feel they've had the same fight several times before.
I have to give it to Supernatural though, it seems they manges to master the art of producing good stand-alone/ monster of the week stories in a better way than Buffy did. For example, in the latest season on Supernatural they tackled the ideas of a fast-food chain being linked to demon activity (7.9 How To Win Friends and Influence Monsters), and a possible supernatural affect of alcohol (7.18 Party On, Garth). Similarly, Buffy tackled similar storylines in the stand-alone episodes (6.12 Doublemeat Palace) and (4.5 Beer Bad) but these episodes are generally perceived as some of the worst in the series, while I didn't mind the Supernatural ones myself.