XTerm. I used to use rxvt-unicode, but it only supports 256 colors and gave me grief when I tried to get some emacs color theme working. There's only one thing I miss, which is that rxvt-unicode reflows lines when you resize the terminal, which xterm won't do. Oh and urxvtc starts very slightly faster, but no big deal.
I also looked at kitty, and I like that the author of that one tries to champion new features, like full keyboard support on par with X11 apps. But it takes noticeably longer to start and the latency also feels worse.