The best replacement for bukkit is spigot.

The best alternatives to bukkit are rated as following:
★★★★★  spigot
★★★★★  forge
★★★★★  craftbukkit
★★★★  vanilla
★★★  tekkit
★★  spout
★★  minecraft server
★  mc server
★  canary
★  hamachi
  hmod
  vanilla server

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

Other search terms related to bukkit and their ratings are:
★★★★★ forge
★★★★ ftb
★★★★ tekkit
★★★ modloader
★★ voltz
★★ minecraft forge
★★ industrial craft
★ mineshafter
★ tekkit lite
You have in mind a companion for bukkit and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of bukkit in all years. Click on graph for more.
bukkit
spigot
forge