ChatZilla has been my favorite client for more than ten years, but its future seems uncertain now that Firefox is moving to WebExtensions only. I like ChatZilla's compromise between simplicity and features.
mIRC used to be the "standard" client on Windows, but I have never been fond of it. Too much stuff.
I tried KVIrc, which crashed twice during the first ten minutes or so. Then Quassel, which couldn't connect to Freenode. Not sure what's going on there.
HexChat is ok, but it tends to forget scrolling the view down when there's a new message. Annoying. And it doesn't have ChatZilla's nice multiline input mode (Ctrl+Up), but I guess I can copy and paste from an editor as a workaround.
There are some nice web based clients, but I feel they are a step down in general.