The best replacement for udk is unity.

The best alternatives to udk are rated as following:
★★★★★  unity
★★★  cryengine
★★  unity 3d
★★  unreal engine 3
★★  source
★★  unreal engine
★  unreal engine 4
★  ue4
★  unity 4
★  cryengine 3
★  cryengine 3 sdk
★  torque 3d
★  unity graphics
★  unreal 3
★  source sdk
  cry engine
  ut3
  unreal
  unity 3
  sdk
  xna
  cry
  torque3d
  unity performance
  unity3d

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

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



Google Trends

Scale is based on the average worldwide traffic of udk in all years. Click on graph for more.
udk
unity
cryengine