The only anime I've bothered watching to the end. It was great. There isn't a single bad character (except for maybe some of the people from Chapter Black, I didn't think they were developed enough). Seasons 1 and 2 are perfect (for different reasons, 1 has the perfect blend of action and drama, while 2 is just awesome fight after awesome fight for 60 episodes). Chapter Black ultimately wasn't that bad, but I just thought that the Sensui fight was way too long, which left a bad taste in my mouth. Three Kings isn't the best but it's short and sweet, in addition to having an awesome ending.
Yusuke and Kuwabara are both great (I slightly prefer Kuwabara) but Kurama is easily the best of the main four. He's one of the few main anime characters I know that is intelligent, he's more merciless than Hiei, and he's ridiculously powerful. I love how he's kind to those who deserve it but will have zero regrets about killing those who don't deserve kindness. Hiei isn't bad either but I don't think he gets developed enough until Season 4, but when he does get there he's great. Even all the supporting characters are great (Koenma and George are the best!).
My favorite fights are the Teguro fight (for obvious reasons) and that fight against the guy who could clone himself in Season 1, whose name escapes me (I just love how it's the time when Yusuke uses literally every ounce of his power that he had at the time, plus Resident Evil: Keiko and Botan Edition was interspersed in-between).
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SuperNiceDog hasn't won yet.