The best replacement for ipad air is ipad mini.

The best alternatives to ipad air are rated as following:
★★★★★  ipad mini
★★★★  ipad 4
★★★★  ipad 2
★★★  ipad
★★  macbook air
★★  kindle fire hdx
★★  ipad mini retina
★★  surface 2
★★  ipad 3
★★  ipad retina
★  samsung
★  ipad mini with retina display
★  nexus 10
★  nexus 7
★  ipad mini 2
★  laptop
★  galaxy note 10.1
★  samsung galaxy note
★  kindle fire
★  mini
★  surface pro 2
  samsung note 10.1 2014
  galaxy 10.1
  ipad with retina display
  samsung galaxy
  galaxy tab pro
  ipad 4th generation
  samsung galaxy tab
  kindle hdx tablet
  note 10.1
  4th generation
  kindle fire hd
  microsoft surface 2
  galaxy note
  galaxy tab
  galaxy tab 3
  galaxy note 10.1 2014
  samsung note
  google nexus 10
  ipad mini 2013
  kindle hdx
  kindle hdx 8.9

Your alternative for ipad air is not listed? Suggest!
Speaking about ipad air, one probably shouldn't forget about retina.

Other search terms related to ipad air and their ratings are:

★★★★★ retina
★★★★ case
You have in mind a companion for ipad air and it is not yet here? Propose!

Google Trends

Scale is based on the average worldwide traffic of ipad air in all years. Click on graph for more.
ipad air
ipad mini
ipad 4