If you're wondering what's the best xp grinder on a skyblock, faction or even on singleplayer, this is for you.

Chicken
Pig
Rabbit
Squid
Cow (and Mooshroom)
Sheep      
Wolf
Ocelot
Horse (and Donkey)                               1-3 XP

Bat 
Villager (when killed)
Iron Golem
Snow Golem       0 XP

Spider
Cave Spider
Creeper
Enderman  
Ghast    
Silverfish   
Skeleton 
Wither Skeleton
Witch
Zombie (Adult)
Pigman  (Adult)               5 XP

Blaze 
Guardian      10 XP

Baby Pigman
Baby Zombie     12 XP

Magma Cube
Slime             4 (Big)  2 (Small) 1 (Tiny)

Wither     50 XP

Ender Dragon        20,000 XP

Conclusion is that on a skyblock, a Blaze spawner is the best.