The best replacement for xchat is mirc.

The best alternatives to xchat are rated as following:
★★★★★  mirc
★★★  pidgin
★★★  hexchat
★★  xchat-gnome
★★  irssi
★  chatzilla
★  colloquy
★  weechat
★  smuxi
  quassel

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

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



Google Trends

Scale is based on the average worldwide traffic of xchat in all years. Click on graph for more.
xchat
mirc
pidgin