The best replacement for activiti is jbpm.

The best alternatives to activiti are rated as following:
★★★★★  jbpm
★★★★  camunda
★★★★  bonita
★★★  jbpm5
★★★  oracle bpm
★★  intalio
★★  processmaker
★★  drools
★★  jbpm 6
★  pega
★  joget
★  drools flow
★  lombardi
  kaleo
  bonitasoft

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

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



Google Trends

Scale is based on the average worldwide traffic of activiti in all years. Click on graph for more.
activiti
jbpm
camunda