Imho, all of the above sites have their good and bad points. Asking this question on Hashnode will of course make Hashnode stand out. You ask people who are here because they like the platform. Ask the same question on Reddit, and people will tell you how great Reddit is. That's why I don't think the poll result is useful at all.
But then again, what's the coolest site? For me, the community which I feel at home is the coolest one.
Also, which one is the coolest for a chat? I'd vote none. They are all discussion-based, but a chat? Chats on Hashnode always ended in endless big reply posts, lacking all the goodies of the full editor, and quickly resembling an email correspondence (and don't get me started about the DMs and the message system, they are nice for quick 1:1 stuff, but not for bigger groups or anything ordered). DevTo is just as messy. Reddit will fork off into side-discussions, which doesn't help at all. And HN is also rather difficult. They don't have a focus on chat. And that's good.
I'll gladly go to a dev server on Discord for a chat, though 😉