The best replacement for reminders is wunderlist.

The best alternatives to reminders are rated as following:
★★★★★  wunderlist
★★★★★  calendar
★★★★  things
★★★  clear
★★  omnifocus
★★  notes
★★  notes iphone
★★  any.do
★★  todoist
★★  calendar iphone
★★  tasks
★  2do
★  todo
★  google tasks
★  calendar ios 5
★  calendar ios
  due
  clear app
  remember the milk
  reminder
  toodledo

Your alternative for reminders is not listed? Suggest!
Speaking about reminders, one probably shouldn't forget about google tasks.

Other search terms related to reminders and their ratings are:
★★★★★ google tasks
★★★★ outlook tasks
★★★ outlook open
You have in mind a companion for reminders and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of reminders in all years. Click on graph for more.
reminders
wunderlist
calendar