The best replacement for ack is grep.

The best alternatives to ack are rated as following:
★★★★★  grep
★★★  nak
★★  nack
★★  ag
★  awk
★  ckc
★  rst
★  ack-grep
  silver searcher
  ack

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

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



Google Trends

Scale is based on the average worldwide traffic of ack in all years. Click on graph for more.
ack
grep
nak