This question is too broad. "Coolest site for developer chat."
What does that mean? How do you define cool? This may sound like I'm being too analytical but these types of things matter. I'll be honest, if I'm looking for a technical answer for a complicated C issue, I'll be going to stack overflow, cboard.cprogramming.com, or another similar site.
If I'm looking for a technical answer to a Windows Driver programming related issue, I'll be going to OSR Online forums.
If I want to find an opinion or subjective answer to something from the dev community or anything web development related, I'll ask it here on Hashnode. Some exceptions apply.. Like I know @maruru is a Rust guy so I sometimes will talk Rust here too.
I think that Hashnode wanted to be the former as well, but I've noticed that this site does tend to be very web-developer heavy and I can't get as much expertise and in-depth discussion going on subjects like C programming, driver programming, low-level network programming, assembly programming, etc as I can elsewhere, so I would say it's not so "cool" for those things.
It would be interesting to hear whether or not the creators of Hashnode such as Sandeep Panda envisioned this as more of a technical site aimed solely at web development or if they do want to take measures to increase the technical depth of the non-web related content here. I think it would be super cool if we could have the aforementioned items here at Hashnode so that I did not need to go to other sites, but right now, that's how I view it. All that said, I chose Hashnode for the poll because I like Hashnode a lot better than those other sites.
Reddit is pretty awful IMO. Every subreddit has its own bullshit rules (You can only ask questions on Mondays after 5pm when the moon is out.... wtf) and moderation staff and you end up accidentally breaking rules, getting your stuff deleted, getting yelled at, when all you wanted to do was ask a simple question.
I don't have time to read 200 different people's sets of rules and frankly the value on the site isn't worth it to begin with. Sorry Reddit.