The best replacement for user controls is custom controls.

The best alternatives to user controls are rated as following:
★★★★★  custom controls
★★  master pages
★  web parts
★  composite controls
★  server controls
★  web custom controls
  web controls

Your alternative for user controls is not listed? Suggest!
Speaking about user controls, one probably shouldn't forget about delegates.

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



Google Trends

Scale is based on the average worldwide traffic of user controls in all years. Click on graph for more.
user controls
custom controls
master pages