The best replacement for tmux is screen.

The best alternatives to tmux are rated as following:
★★★★★  screen
★★★★  terminator
★★★  byobu
★★★  iterm2
★★  xmonad
★★  gnu screen
★★  dvtm
★★  mosh
★  nohup

Your alternative for tmux is not listed? Suggest!
Speaking about tmux, one probably shouldn't forget about iterm2.

Other search terms related to tmux and their ratings are:
★★★★★ iterm2
★★★★★ vim
★★ irssi
You have in mind a companion for tmux and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of tmux in all years. Click on graph for more.
tmux
screen
terminator