The best replacement for tyranids is zerg.

The best alternatives to tyranids are rated as following:
★★★★★  zerg
★★★★  necrons
★★★  space marines
★★★  orks
★★★  tau
★★  chaos
★★  eldar
★★  dark eldar
★  grey knights
★  flood
★  chaos space marines
★  reapers
★  necromorphs
  space wolves
  tau tactics
  imperial guard
  dark eldar tactics
  daemons
  chaos daemons
  xenomorphs
  necron
  forerunners

Your alternative for tyranids is not listed? Suggest!
Speaking about tyranids, one probably shouldn't forget about number rule.

Other search terms related to tyranids and their ratings are:
★★★★★ number rule
★★★★ space marines
★★★ necrons
You have in mind a companion for tyranids and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of tyranids in all years. Click on graph for more.
tyranids
zerg
necrons