Which plugin?

[GlitchGalore]

Stackmob
https://www.spigotmc.org/resources/stackmob.29999/
or
MergedMob
https://www.spigotmc.org/resources/mergedmob-with-custom-mob-drops-configuration.11399/
or
MobStacker (idk where mobstacker2 went)
https://www.spigotmc.org/resources/mobstacker.15596/

 

[ArZor]

Well considering MobStacker is outdated and no longer receiving support and MobStacker2 has a broken link i'd imagine that one being out.
Then i'd say you have to ask yourself would you use all the features of MergedMob to justify the purchase since it is a premium resource.
If not i'd say StackMob is your winner.

 

[Matthew_Cash]

Stackmob
https://www.spigotmc.org/resources/stackmob.29999/
or
MergedMob
https://www.spigotmc.org/resources/mergedmob-with-custom-mob-drops-configuration.11399/
or
MobStacker (idk where mobstacker2 went)
https://www.spigotmc.org/resources/mobstacker.15596/

I have used MergedMob in the past, for me, it honestly was not worth the money. It's really up to you